net: bridge: mcast: add support for src list and filter mode dumping

Support per port group src list (address and timer) and filter mode
dumping. Protected by either multicast_lock or rcu.

v3: add IPv6 support
v2: require RCU or multicast_lock to traverse src groups

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>