[NETLINK]: Fix multicast bind/autobind race

netlink_autobind has always set nlk_sk(sk)->groups to zero.  This is
unnecessary because sk_alloc already zeroes the entire structure.
Since a socket can only be bound once netlink_autobind doesn't need
to zero groups at all.

This had been safe until I added mc_list.  Now it is possible for
netlink_bind to race against netlink_autobind running on the same
socket on another CPU.  The result would be a socket that's on
mc_list with groups set to zero.  This socket will be left on the
list even after it is destroyed.

The fix is to remove the zeroing in netlink_autobind.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -430,7 +430,6 @@ static int netlink_autobind(struct socket *sock)
 	err = netlink_insert(sk, pid);
 	if (err == -EADDRINUSE)
 		goto retry;
-	nlk_sk(sk)->groups = 0;
 	return 0;
 }