[NETLINK]: negative groups in netlink_setsockopt

Reading netlink_setsockopt it's not immediately clear why there isn't a
bug when you pass in negative numbers, the reason being that the >=
comparison is really unsigned although 'val' is signed because
nlk->ngroups is unsigned. Make 'val' unsigned too.

[ Update the get_user() cast to match.  --DaveM ]

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>