[TCP]: Forward port of 2.4.x bugfix, noticed as missing by davej@codemonkey.org.uk.

In tcp_sendmsg, make sure we jump to the out label
when seglen is decremented to zero and no more iovecs remain.
This matches the do_tcp_sendpages logic and makes sure that
PSH is set correctly at the end of a write even if the write length
equals the current mss.

net/ipv4/tcp.c

@@ -1189,7 +1189,8 @@ int tcp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 
 			from += copy;
 			copied += copy;
-			seglen -= copy;
+			if ((seglen -= copy) == 0 && iovlen == 0)
+				goto out;
 
 			if (skb->len != mss_now || (flags & MSG_OOB))
 				continue;