[NETLINK]: cb_lock does not needs ref count on sk

Here is a little optimisation for the cb_lock used by netlink_dump.
While fixing that race earlier, I noticed that the reference count
held by cb_lock is completely useless.  The reason is that in order
to obtain the protection of the reference count, you have to take
the cb_lock.  But the only way to take the cb_lock is through
dereferencing the socket.

That is, you must already possess a reference count on the socket
before you can take advantage of the reference count held by cb_lock.
As a corollary, we can remve the reference count held by the cb_lock.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -373,7 +373,6 @@ static int netlink_release(struct socket *sock)
 		nlk->cb->done(nlk->cb);
 		netlink_destroy_callback(nlk->cb);
 		nlk->cb = NULL;
-		__sock_put(sk);
 	}
 	spin_unlock(&nlk->cb_lock);
 
@@ -1099,7 +1098,6 @@ static int netlink_dump(struct sock *sk)
 	spin_unlock(&nlk->cb_lock);
 
 	netlink_destroy_callback(cb);
-	__sock_put(sk);
 	return 0;
 }
 
@@ -1138,7 +1136,6 @@ int netlink_dump_start(struct sock *ssk, struct sk_buff *skb,
 		return -EBUSY;
 	}
 	nlk->cb = cb;
-	sock_hold(sk);
 	spin_unlock(&nlk->cb_lock);
 
 	netlink_dump(sk);