bridge: Move locking defines into if_bridge.c

The locking defines for if_bridge used to live in if_bridgevar.h, but
they're only ever used by the bridge implementation itself (in
if_bridge.c). Moving them into the .c file.

Reported by: philip, emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23808