net: bridge: vlan: add mcast snooping control

Add a new global vlan option which controls whether multicast snooping
is enabled or disabled for a single vlan. It controls the vlan private
flag: BR_VLFLAG_GLOBAL_MCAST_ENABLED.

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