bridge: add MDB state mask uAPI attribute

Currently, the 'state' field in 'struct br_port_msg' can be set to 1 if
the MDB entry is permanent or 0 if it is temporary. Additional states
might be added in the future.

In a similar fashion to 'NDA_NDM_STATE_MASK', add an MDB state mask uAPI
attribute that will allow the upcoming bulk deletion API to bulk delete
MDB entries with a certain state or any state.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>