Initial functions for group source lists which are needed for IGMPv3
and MLDv2 include/exclude lists. Both IPv4 and IPv6 sources are supported.
User-added mdb entries are created with exclude filter mode, we can
extend that later to allow user-supplied mode. When group src entries
are deleted, they're freed from a workqueue to make sure their timers
are not still running. Source entries are protected by the multicast_lock
and rcu. The number of src groups per port group is limited to 32.

v4: use the new port group del function directly
    add igmpv2/mldv1 bool to denote if the entry was added in those
    modes, it will later replace the old update_timer bool
v3: add IPv6 support
v2: allow src groups to be traversed under rcu
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>