Since each entry type has timers that can be running simultaneously we need
to make sure that entries are not freed before their timers have finished.
In order to do that generalize the src gc work to mcast gc work and use a
callback to free the entries (mdb, port group or src).

v3: add IPv6 support
v2: force mcast gc on port del to make sure all port group timers have
    finished before freeing the bridge port
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>