netgraph/ng_bridge: become SMP aware

The node ng_bridge underwent a lot of changes in the last few months.
All those steps were necessary to distinguish between structure
modifying and read-only data transport paths. Now it's done, the node
can perform frame forwarding on multiple cores in parallel.

Differential Revision: https://reviews.freebsd.org/D28123

(cherry picked from commit 9674c2e68c613757ddf5a8780743842b5ac4d74f)