net: bridge: mcast: add sg_port rhashtable

To speedup S,G forward handling we need to be able to quickly find out
if a port is a member of an S,G group. To do that add a global S,G port
rhashtable with key: source addr, group addr, protocol, vid (all br_ip
fields) and port pointer.

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