net: bridge: vlan: enable mcast snooping for existing master vlans

We always create a vlan with enabled mcast snooping, so when the user
turns on per-vlan mcast contexts they'll get consistent behaviour with
the current situation, but one place wasn't updated when a bridge/master
vlan which already exists (created due to port vlans) is being added as
real bridge vlan (BRIDGE_VLAN_INFO_BRENTRY). We need to enable mcast
snooping for that vlan when that happens.

Fixes: 7b54aaaf ("net: bridge: multicast: add vlan state initialization and control")
Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>