tcp: enhance tcp collapsing

As Ilya Lesokhin suggested, we can collapse two skbs at retransmit
time even if the skb at the right has fragments.

We simply have to use more generic skb_copy_bits() instead of
skb_copy_from_linear_data() in tcp_collapse_retrans()

Also need to guard this skb_copy_bits() in case there is nothing to
copy, otherwise skb_put() could panic if left skb has frags.

Tested:

Used following packetdrill test

// Establish a connection.
0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
   +0 bind(3, ..., ...) = 0
   +0 listen(3, 1) = 0

   +0 < S 0:0(0) win 32792 <mss 1460,sackOK,nop,nop,nop,wscale 8>
   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
+.100 < . 1:1(0) ack 1 win 257
   +0 accept(3, ..., ...) = 4

   +0 setsockopt(4, SOL_TCP, TCP_NODELAY, [1], 4) = 0
   +0 write(4, ..., 200) = 200
   +0 > P. 1:201(200) ack 1
+.001 write(4, ..., 200) = 200
   +0 > P. 201:401(200) ack 1
+.001 write(4, ..., 200) = 200
   +0 > P. 401:601(200) ack 1
+.001 write(4, ..., 200) = 200
   +0 > P. 601:801(200) ack 1
+.001 write(4, ..., 200) = 200
   +0 > P. 801:1001(200) ack 1
+.001 write(4, ..., 100) = 100
   +0 > P. 1001:1101(100) ack 1
+.001 write(4, ..., 100) = 100
   +0 > P. 1101:1201(100) ack 1
+.001 write(4, ..., 100) = 100
   +0 > P. 1201:1301(100) ack 1
+.001 write(4, ..., 100) = 100
   +0 > P. 1301:1401(100) ack 1

+.100 < . 1:1(0) ack 1 win 257 <nop,nop,sack 1001:1401>
// Check that TCP collapse works :
   +0 > P. 1:1001(1000) ack 1
Reported-by: Ilya Lesokhin <ilyal@mellanox.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -2529,7 +2529,8 @@ static void tcp_collapse_retrans(struct sock *sk, struct sk_buff *skb)
 
 	tcp_unlink_write_queue(next_skb, sk);
 
-	skb_copy_from_linear_data(next_skb, skb_put(skb, next_skb_size),
+	if (next_skb_size)
+		skb_copy_bits(next_skb, 0, skb_put(skb, next_skb_size),
 			      next_skb_size);
 
 	if (next_skb->ip_summed == CHECKSUM_PARTIAL)
@@ -2567,14 +2568,11 @@ static bool tcp_can_collapse(const struct sock *sk, const struct sk_buff *skb)
 {
 	if (tcp_skb_pcount(skb) > 1)
 		return false;
-	/* TODO: SACK collapsing could be used to remove this condition */
-	if (skb_shinfo(skb)->nr_frags != 0)
-		return false;
 	if (skb_cloned(skb))
 		return false;
 	if (skb == tcp_send_head(sk))
 		return false;
-	/* Some heurestics for collapsing over SACK'd could be invented */
+	/* Some heuristics for collapsing over SACK'd could be invented */
 	if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
 		return false;