[IPV4]: Possible leak of multicast source filter sctructure

There is a leak of a socket's multicast source filter list structure
on closing a socket with a multicast source filter set on an interface
that does not exist any more.

Signed-off-by: Michal Ruzicka <michal.ruzicka@comstar.cz>
Acked-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>