net/sock: add WARN_ON(parent->sk) in sock_graft()

sock_graft() unilaterally sets up parent->sk based on the
assumption that the existing parent->sk is null. If this
condition is not true, then the existing parent->sk would
be leaked, so add a WARN_ON() to alert callers who may fall
in this category.
Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -1697,6 +1697,7 @@ static inline void sock_orphan(struct sock *sk)
 
 static inline void sock_graft(struct sock *sk, struct socket *parent)
 {
+	WARN_ON(parent->sk);
 	write_lock_bh(&sk->sk_callback_lock);
 	sk->sk_wq = parent->wq;
 	parent->sk = sk;