CA-127274: Linux bridge: set forwarding delay to 0 after creating a bridge

The forwarding delay is 30s by default, which means that for the first 30s
after bringing up a bridge port, the port will be in "learning mode", and won't
forward any packets. We want bridge ports to immediately go in "forwarding
mode" instead.

We did this in the past, but it was lost in the transition to xcp-networkd.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>