net: replace br_fdb_external_learn_* calls with switchdev notifier events

This patch benefits from newly introduced switchdev notifier and uses it
to propagate fdb learn events from rocker driver to bridge. That avoids
direct function calls and possible use by other listeners (ovs).

Suggested-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>