[TCP]: Fix sk_forward_alloc underflow in tcp_sendmsg

I've finally found a potential cause of the sk_forward_alloc underflows
that people have been reporting sporadically.

When tcp_sendmsg tacks on extra bits to an existing TCP_PAGE we don't
check sk_forward_alloc even though a large amount of time may have
elapsed since we allocated the page.  In the mean time someone could've
come along and liberated packets and reclaimed sk_forward_alloc memory.

This patch makes tcp_sendmsg check sk_forward_alloc every time as we
do in do_tcp_sendpages.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -1232,9 +1232,8 @@ static inline struct page *sk_stream_alloc_page(struct sock *sk)
 {
 	struct page *page = NULL;
 
-	if (sk_stream_wmem_schedule(sk, PAGE_SIZE))
-		page = alloc_pages(sk->sk_allocation, 0);
-	else {
+	page = alloc_pages(sk->sk_allocation, 0);
+	if (!page) {
 		sk->sk_prot->enter_memory_pressure();
 		sk_stream_moderate_sndbuf(sk);
 	}

net/ipv4/tcp.c

@@ -769,19 +769,23 @@ int tcp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 					if (off == PAGE_SIZE) {
 						put_page(page);
 						TCP_PAGE(sk) = page = NULL;
+						TCP_OFF(sk) = off = 0;
 					}
-				}
+				} else
+					BUG_ON(off);
+
+				if (copy > PAGE_SIZE - off)
+					copy = PAGE_SIZE - off;
+
+				if (!sk_stream_wmem_schedule(sk, copy))
+					goto wait_for_memory;
 
 				if (!page) {
 					/* Allocate new cache page. */
 					if (!(page = sk_stream_alloc_page(sk)))
 						goto wait_for_memory;
-					off = 0;
 				}
 
-				if (copy > PAGE_SIZE - off)
-					copy = PAGE_SIZE - off;
-
 				/* Time to copy data. We are close to
 				 * the end! */
 				err = skb_copy_to_page(sk, from, skb, page,