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

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