net: Several cleanups for the setsockopt compat support.

1) added missing "__user" for kgsr and kgf pointers
2) verify read for only GROUP_FILTER_SIZE(0). The group_filter
        structure definition (via RFC) includes space for one source
        in the source list array, but that source need not be present.
        So, sizeof(group_filter) > GROUP_FILTER_SIZE(0). Fixed
        the user read-check for minimum length to use the smaller size.
3) remove unneeded "&" for gf_slist addresses

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>