Fix Linux optimizations (hassio-addons#360)

tailscale/rootfs/etc/s6-overlay/s6-rc.d/post-tailscaled/run

@@ -4,6 +4,7 @@
 # Home Assistant Community Add-on: Tailscale
 # Runs after the machine has been logged in into the Tailscale network
 # ==============================================================================
+declare default_route_device
 declare -a options
 declare -a routes=()
 declare route
@@ -16,7 +17,10 @@ declare keyexpiry
 # Based on: https://tailscale.com/kb/1320/performance-best-practices#linux-optimizations-for-subnet-routers-and-exit-nodes
 # Note: Changes made via ethtool are not persistent and will be lost after the machine shuts down.
 # Note: Executing it before "tailscale up" to avoid warning messages
-ethtool -K $(ip route show 0/0 | cut -f5 -d' ') rx-udp-gro-forwarding on rx-gro-list off
+default_route_device=$(ip -4 route show 0/0 | cut -f5 -d' ')
+if bashio::var.has_value "${default_route_device}"; then
+  ethtool -K "${default_route_device}" rx-udp-gro-forwarding on rx-gro-list off
+fi
 
 # Default options
 options+=(--hostname "$(bashio::info.hostname)")