net/8021q: Implement Multiple VLAN Registration Protocol (MVRP)

Initial implementation of the Multiple VLAN Registration Protocol
(MVRP) from IEEE 802.1Q-2011, based on the existing implementation
of the GARP VLAN Registration Protocol (GVRP).

Signed-off-by: David Ward <david.ward@ll.mit.edu>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>