bridge: add new brport flag LEARNING_SYNC

This policy flag controls syncing of learned FDB entries to bridge's FDB.  If
on, FDB entries learned on bridge port device will be synced.  If off, device
may still learn new FDB entries but they will not be synced with bridge's FDB.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: Andy Gospodarek <gospo@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>