ipv4: add a sock pointer to ip_queue_xmit()

ip_queue_xmit() assumes the skb it has to transmit is attached to an
inet socket. Commit 31c70d5956fc ("l2tp: keep original skb ownership")
changed l2tp to not change skb ownership and thus broke this assumption.

One fix is to add a new 'struct sock *sk' parameter to ip_queue_xmit(),
so that we do not assume skb->sk points to the socket used by l2tp
tunnel.

Fixes: 31c70d5956fc ("l2tp: keep original skb ownership")
Reported-by: Zhan Jianyu <nasa4836@gmail.com>
Tested-by: Zhan Jianyu <nasa4836@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/l2tp/l2tp_ip.c b/net/l2tp/l2tp_ip.c
index 0b44d85..3397fe6 100644
--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -487,7 +487,7 @@
 
 xmit:
 	/* Queue the packet to IP for output */
-	rc = ip_queue_xmit(skb, &inet->cork.fl);
+	rc = ip_queue_xmit(sk, skb, &inet->cork.fl);
 	rcu_read_unlock();
 
 error: