netgraph/ng_bridge: learn MACs via control message

Add a new control message to move ethernet addresses to a given link
in ng_bridge(4). Send this message instead of doing the work directly.
This decouples the read-only activity from the modification under a
more strict writer lock.

Decoupling the work is a prerequisite for multithreaded operation.

Approved by: manpages (bcr), kp (earlier version)
MFC: 3 weeks
Differential Revision: https://reviews.freebsd.org/D28516