[MCAST]: Fix filter leak on device removal.

This fixes source filter leakage when a device is removed and a
process leaves the group thereafter.

This also includes corresponding fixes for IPv6 multicast source
filters on device removal.

Signed-off-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>