tcp: better use ooo_last_skb in tcp_data_queue_ofo()

Willem noticed that we could avoid an rbtree lookup if the
the attempt to coalesce incoming skb to the last skb failed
for some reason.

Since most ooo additions are at the tail, this is definitely
worth adding a test and fast path.

Suggested-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yaogong Wang <wygivan@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -4461,6 +4461,12 @@ coalesce_done:
 		skb = NULL;
 		goto add_sack;
 	}
+	/* Can avoid an rbtree lookup if we are adding skb after ooo_last_skb */
+	if (!before(seq, TCP_SKB_CB(tp->ooo_last_skb)->end_seq)) {
+		parent = &tp->ooo_last_skb->rbnode;
+		p = &parent->rb_right;
+		goto insert;
+	}
 
 	/* Find place to insert this segment. Handle overlaps on the way. */
 	parent = NULL;
@@ -4503,7 +4509,7 @@ coalesce_done:
 		}
 		p = &parent->rb_right;
 	}
-
+insert:
 	/* Insert segment into RB tree. */
 	rb_link_node(&skb->rbnode, parent, p);
 	rb_insert_color(&skb->rbnode, &tp->out_of_order_queue);