[TCP]: Abstract out all write queue operations.

This allows the write queue implementation to be changed,
for example, to one which allows fast interval searching.

Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -1890,7 +1890,7 @@ int tcp_v4_destroy_sock(struct sock *sk)
 	tcp_cleanup_congestion_control(sk);
 
 	/* Cleanup up the write buffer. */
-	sk_stream_writequeue_purge(sk);
+	tcp_write_queue_purge(sk);
 
 	/* Cleans up our, hopefully empty, out_of_order_queue. */
 	__skb_queue_purge(&tp->out_of_order_queue);