net: bridge: mcast: install S,G entries automatically based on reports

This patch adds support for automatic install of S,G mdb entries based
on the port group's source list and the source entry's timer.
Once installed the S,G will be used when forwarding packets if the
approprate multicast/mld versions are set. A new source flag called
BR_SGRP_F_INSTALLED denotes if the source has a forwarding mdb entry
installed.

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