ipv6: Compilation fix for compat MCAST_MSFILTER sockopts.

The last hunk from the commit dae50295 (ipv4/ipv6 compat: Fix SSM
applications on 64bit kernels.) escaped from the compat_ipv6_setsockopt
to the ipv6_getsockopt (I guess due to patch smartness wrt searching
for context) thus breaking 32-bit and 64-bit-without-compat compilation.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>