Finish r191148: replace rtentry with route in if_bridge if_output() callback.

Generic if_output() callback signature was modified to use struct route
instead of struct rtentry in r191148, back in 2009.

Quoting commit message:

Change if_output to take a struct route as its fourth argument in order
to allow passing a cached struct llentry * down to L2

Fix bridge_output() to match this signature and update the remaining
comment in if_var.h.

Reviewed by: kp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24394