if_bridge(4): ensure all traffic passing over the bridge is accounted for

Consider a bridge0 with em0 and em1 members. Traffic rx'd by em0 and
transmitted by bridge0 through em1 gets accounted for in IPACKETS/IBYTES
and bridge0 bpf -- assuming it's not unicast traffic destined for em1.
Unicast traffic destined for em1 traffic is not accounted for by any
mechanism, and isn't pushed through bridge0's bpf machinery as any other
packets that pass over the bridge do.

Fix this and simplify GRAB_OUR_PACKETS by bailing out early if it was rx'd
by the interface that it was addressed for. Everything else there is
relevant for any traffic that came in from one member that's being directed
at another member of the bridge.

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D19614