Add "delete" command to "rootfs-container" script

This change extends the "rootfs-container" script to enable the deletion
of existing rootfs containers. For example, given the following system:

    $ sudo zfs list
    NAME                              USED  AVAIL     REFER  MOUNTPOINT
    rpool                            38.5G  28.9G       64K  none
    rpool/ROOT                       34.0G  28.9G       64K  none
    rpool/ROOT/delphix.739W0dx       5.56G  28.9G       64K  none
    rpool/ROOT/delphix.739W0dx/data   840K  28.9G     27.9M  legacy
    rpool/ROOT/delphix.739W0dx/home   155K  28.9G     11.0G  legacy
    rpool/ROOT/delphix.739W0dx/root  5.56G  28.9G     5.56G  /
    rpool/ROOT/delphix.BTC02Vz       5.98G  28.9G       65K  none
    rpool/ROOT/delphix.BTC02Vz/data  3.32M  28.9G     29.1M  legacy
    rpool/ROOT/delphix.BTC02Vz/home   346K  28.9G     11.0G  legacy
    rpool/ROOT/delphix.BTC02Vz/root  5.98G  28.9G     5.97G  /
    rpool/ROOT/delphix.fuzt21y       5.54G  28.9G       64K  none
    rpool/ROOT/delphix.fuzt21y/data   620K  28.9G     28.5M  legacy
    rpool/ROOT/delphix.fuzt21y/home   152K  28.9G     11.0G  legacy
    rpool/ROOT/delphix.fuzt21y/root  5.54G  28.9G     5.54G  /
    rpool/ROOT/delphix.ns2t0fx       16.9G  28.9G       64K  none
    rpool/ROOT/delphix.ns2t0fx/data  29.8M  28.9G     29.0M  legacy
    rpool/ROOT/delphix.ns2t0fx/home  11.0G  28.9G     11.0G  legacy
    rpool/ROOT/delphix.ns2t0fx/root  5.89G  28.9G     5.88G  /
    rpool/crashdump                  65.5K  17.4G     65.5K  /var/crash
    rpool/grub                       3.03M  28.9G     3.03M  legacy
    rpool/update                     4.48G  10.5G     4.48G  /var/dlpx-update

We can now use this new command to remove all but the currently booted
rootfs container:

    $ sudo /var/dlpx-update/latest/rootfs-container delete delphix.BTC02Vz
    $ sudo /var/dlpx-update/latest/rootfs-container delete delphix.739W0dx
    $ sudo /var/dlpx-update/latest/rootfs-container delete delphix.ns2t0fx

    $ zfs list -t all
    NAME                                              USED  AVAIL     REFER  MOUNTPOINT
    rpool                                            21.1G  46.3G       64K  none
    rpool/ROOT                                       16.6G  46.3G       64K  none
    rpool/ROOT/delphix.fuzt21y                       16.6G  46.3G       64K  none
    rpool/ROOT/delphix.fuzt21y/data                  30.4M  46.3G     28.9M  legacy
    rpool/ROOT/delphix.fuzt21y/[email protected]   958K      -     27.4M  -
    rpool/ROOT/delphix.fuzt21y/[email protected]   298K      -     27.4M  -
    rpool/ROOT/delphix.fuzt21y/[email protected]   336K      -     28.2M  -
    rpool/ROOT/delphix.fuzt21y/home                  11.0G  46.3G     11.0G  legacy
    rpool/ROOT/delphix.fuzt21y/[email protected]   152K      -     11.0G  -
    rpool/ROOT/delphix.fuzt21y/[email protected]   143K      -     11.0G  -
    rpool/ROOT/delphix.fuzt21y/[email protected]   143K      -     11.0G  -
    rpool/ROOT/delphix.fuzt21y/root                  5.58G  46.3G     5.58G  /
    rpool/crashdump                                  65.5K  17.4G     65.5K  /var/crash
    rpool/grub                                       3.03M  46.3G     3.03M  legacy
    rpool/update                                     4.48G  10.5G     4.48G  /var/dlpx-update

Additionally, it'll prevent the user from deleting the rootfs container
that's currently booted:

    $ zfs list /
    NAME                              USED  AVAIL     REFER  MOUNTPOINT
    rpool/ROOT/delphix.fuzt21y/root  5.57G  46.3G     5.57G  /

    $ sudo /var/dlpx-update/latest/rootfs-container delete delphix.fuzt21y
    rootfs-container: cannot delete mounted rootfs container: 'delphix.fuzt21y'

upgrade/upgrade-scripts/rootfs-container

@@ -19,6 +19,43 @@
 
 CONTAINER=
 
+function get_dataset_snapshots() {
+	zfs list -rt snapshot -Hpo name "$1"
+}
+
+function get_snapshot_clones() {
+	zfs get clones -Hpo value "$1"
+}
+
+function delete() {
+	zfs list "rpool/ROOT/$CONTAINER/root" &>/dev/null ||
+		die "rootfs container '$CONTAINER' does not exist"
+
+	MOUNTED=$(zfs get mounted -Hpo value "rpool/ROOT/$CONTAINER/root")
+	[[ "$MOUNTED" == "no" ]] ||
+		die "cannot delete mounted rootfs container: '$CONTAINER'"
+
+	#
+	# The "data" and "home" datasets of a rootfs container may have
+	# been cloned as part of an upgrade. Thus, in order to delete
+	# this specific rootfs container, we need to promote any clones
+	# that exist. Otherwise we won't be able to destroy the snapshots
+	# for the "data" and "home" datasets, and thus can't destroy the
+	# datasets themselves.
+	#
+	for snap in \
+		$(get_dataset_snapshots "rpool/ROOT/$CONTAINER/data") \
+		$(get_dataset_snapshots "rpool/ROOT/$CONTAINER/home"); do
+		for clone in $(get_snapshot_clones "$snap"); do
+			zfs promote "$clone" ||
+				die "'zfs promote $clone' failed"
+		done
+	done
+
+	zfs destroy -r "rpool/ROOT/$CONTAINER" ||
+		die "'zfs destroy -r rpool/ROOT/$CONTAINER' failed"
+}
+
 function get_bootloader_devices() {
 	#
 	# When installing/updating the bootloader during upgrade, we
@@ -184,6 +221,7 @@ function usage() {
 	PREFIX_NCHARS=$(echo -n "$PREFIX_STRING" | wc -c)
 	PREFIX_SPACES=$(printf "%.s " $(seq "$PREFIX_NCHARS"))
 
+	echo "$PREFIX_SPACES delete <container>"
 	echo "$PREFIX_SPACES set-bootfs <container>"
 
 	exit 2
@@ -192,6 +230,13 @@ function usage() {
 [[ "$EUID" -ne 0 ]] && die "must be run as root"
 
 case "$1" in
+delete)
+	[[ $# -lt 2 ]] && usage "too few arguments specified"
+	[[ $# -gt 2 ]] && usage "too many arguments specified"
+
+	CONTAINER="$2"
+	delete
+	;;
 set-bootfs)
 	[[ $# -lt 2 ]] && usage "too few arguments specified"
 	[[ $# -gt 2 ]] && usage "too many arguments specified"