Rollback before zfs root is mounted

On my machines I observe random failures caused by rollback happening 
after zfs root is mounted. I've observed two types of failures:

- zfs-rollback-bootfs.service fails saying that rollback must be
  done just before mounting the dataset
- boot process fails and rescue console is entered.

After making this modification and testing it for couple of days 
none of those problems have been observed anymore.

I don't know if `dracut-mount.service` is still needed in the 
`After` directive. Maybe someone else is able to address this?

Reviewed-by: Gregory Bartholomew <[email protected]>
Signed-off-by: Wojciech Małota-Wójcik <[email protected]>
Closes #15025

contrib/dracut/90zfs/zfs-rollback-bootfs.service.in

@@ -2,7 +2,7 @@
 Description=Rollback bootfs just before it is mounted
 Requisite=zfs-import.target
 After=zfs-import.target dracut-pre-mount.service zfs-snapshot-bootfs.service
-Before=dracut-mount.service
+Before=dracut-mount.service sysroot.mount
 DefaultDependencies=no
 ConditionKernelCommandLine=bootfs.rollback
 ConditionEnvironment=BOOTFS