per-socket multicast filters were not being applied to all sockets
in the case of an exact-match bound address, due to an over-exuberant
"return" in the look-up code. Fix below. IPv4 does not have this problem.

Thanks to Hoerdt Mickael for reporting the bug.
Signed-off-by: NDavid L Stevens <dlstevens@us.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>