Finish r191148: replace rtentry with route in if_bridge.

[melifaro]

Summary

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.