net: bridge: multicast: add mdb context support

Choose the proper bridge multicast context when user-spaces is adding
mdb entries. Currently we require the vlan to be configured on at least
one device (port or bridge) in order to add an mdb entry if vlan
mcast snooping is enabled (vlan snooping implies vlan filtering).
Note that we always allow deleting an entry, regardless of the vlan state.

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