net: bridge: mdb: move all port and bridge checks to br_mdb_add

To avoid doing duplicate device checks and searches (the same were done
in br_mdb_add and __br_mdb_add) pass the already found port to __br_mdb_add
and pull the bridge's netif_running and enabled multicast checks to
br_mdb_add. This would also simplify the future extack errors.

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>