tcp: tcp_sendmsg() page recycling

If our TCP_PAGE(sk) is not shared (page_count() == 1), we can set page
offset to 0.

This permits better filling of the pages on small to medium tcp writes.

"tbench 16" results on my dev server (2x4x2 machine) :

Before : 3072 MB/s
After  : 3146 MB/s  (2.4 % gain)
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -1009,7 +1009,12 @@ int tcp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 				int merge = 0;
 				int i = skb_shinfo(skb)->nr_frags;
 				struct page *page = TCP_PAGE(sk);
-				int off = TCP_OFF(sk);
+				int off;
+
+				if (page && page_count(page) == 1)
+					TCP_OFF(sk) = 0;
+
+				off = TCP_OFF(sk);
 
 				if (skb_can_coalesce(skb, i, page, off) &&
 				    off != PAGE_SIZE) {