bridge: Require learning table at all times.

bridge: Require learning table at all times.

The bridge nominally allowed the MAC learning module to not be enabled
though in reality it was always used.  Tracking active MAC addresses
in the bridge is useful for other reasons besides deciding the output
port - primarily for bonding.  In addition there were several bugs
that would have been triggered had learning actually been disabled since
that code path is never tested.  This makes it explicit that the learning
table should be maintained at all times.