net: Flush deferred skb free on socket destroy

The cited Fixes patch moved to a deferred skb approach where the skbs
are not freed immediately under the socket lock.  Add a WARN_ON_ONCE()
to verify the deferred list is empty on socket destroy, and empty it to
prevent potential memory leaks.

Fixes: f35f8219 ("tcp: defer skb freeing after socket lock is released")
Signed-off-by: NGal Pressman <gal@nvidia.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -2049,6 +2049,9 @@ void sk_destruct(struct sock *sk)
 {
 	bool use_call_rcu = sock_flag(sk, SOCK_RCU_FREE);
 
+	WARN_ON_ONCE(!llist_empty(&sk->defer_list));
+	sk_defer_free_flush(sk);
+
 	if (rcu_access_pointer(sk->sk_reuseport_cb)) {
 		reuseport_detach_sock(sk);
 		use_call_rcu = true;