brcompat: Tolerate a race condition in deleting bridge ports.

brcompat: Tolerate a race condition in deleting bridge ports.

When we delete a bridge port (e.g. "brctl delif"), the port could get
removed from the bridge we are interested in and then quickly added back
into another bridge while sleeping.  Return immediately in this case,
since the port must really have been deleted from the bridge in question.

There is a remaining race that the port could get deleted from the bridge
and then added back to the same one.