netlink: Do not subscribe to non-existent groups

Make ABI more strict about subscribing to group > ngroups.
Code doesn't check for that and it looks bogus.
(one can subscribe to non-existing group)
Still, it's possible to bind() to all possible groups with (-1)

Cc: "David S. Miller" <[email protected]>
Cc: Herbert Xu <[email protected]>
Cc: Steffen Klassert <[email protected]>
Cc: [email protected]
Signed-off-by: Dmitry Safonov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: 0x7f454c46

net/netlink/af_netlink.c

@@ -1008,6 +1008,7 @@ static int netlink_bind(struct socket *sock, struct sockaddr *addr,
 		if (err)
 			return err;
 	}
+	groups &= (1UL << nlk->ngroups) - 1;
 
 	bound = nlk->bound;
 	if (bound) {