bridge: mdb: add support for offloaded mdb entries

Add new bitmask member 'flags' to br_mdb_entry structure. Adding
MDB_FLAGS_OFFLOAD bit which indicates MDB entries is offloaded to hardware.

Signed-off-by: Elad Raz <eladr@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>