net: bridge: multicast: check if should use vlan mcast ctx

Add helpers which check if the current bridge/port multicast context
should be used (i.e. they're not disabled) and use them for Rx IGMP/MLD
processing, timers and new group addition. It is important for vlans to
disable processing of timer/packet after the multicast_lock is obtained
if the vlan context doesn't have BR_VLFLAG_MCAST_ENABLED. There are two
cases when that flag is missing:
 - if the vlan is getting destroyed it will be removed and timers will
   be stopped
 - if the vlan mcast snooping is being disabled

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