netgraph/ng_bridge: move MACs via control message

Use the new control message to move ethernet addresses from a link to
a new link in ng_bridge(4). Send this message instead of doing the
work directly requires to move the loop detection into the control
message processing. This will delay the loop detection by a few
frames.

This decouples the read-only activity from the modification under a
more strict writer lock.

Reviewed by: manpages (gbe)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D28559