net: Add compat support for getsockopt (MCAST_MSFILTER)

This patch adds support for getsockopt for MCAST_MSFILTER for
both IPv4 and IPv6. It depends on the previous setsockopt patch,
and uses the same method.

Signed-off-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>