ipv4: do compat setsockopt for MCAST_MSFILTER directly

Parallel to what the native setsockopt() does, except that unlike
the native setsockopt() we do not use memdup_user() - we want
the sockaddr_storage fields properly aligned, so we allocate
4 bytes more and copy compat_group_filter at the offset 4,
which yields the proper alignments.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>