netgraph/ng_bridge: Avoid cache thrashing

Hint the compiler, that this update is needed at most once per second.
Only in this case the memory line needs to be written. This will
reduce the amount of cache trashing during forward of most frames.

Suggested by: zec
Approved by: zec
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D28601