tipc: call tipc_wait_for_connect only when dlen is not 0

__tipc_sendmsg() is called to send SYN packet by either tipc_sendmsg()
or tipc_connect(). The difference is in tipc_connect(), it will call
tipc_wait_for_connect() after __tipc_sendmsg() to wait until connecting
is done. So there's no need to wait in __tipc_sendmsg() for this case.

This patch is to fix it by calling tipc_wait_for_connect() only when dlen
is not 0 in __tipc_sendmsg(), which means it's called by tipc_connect().

Note this also fixes the failure in tipcutils/test/ptts/:

  # ./tipcTS &
  # ./tipcTC 9
  (hang)

Fixes: 36239dab6da7 ("tipc: fix implicit-connect for SYN+")
Reported-by: Shuang Li <shuali@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/socket.c

@@ -1518,7 +1518,7 @@ static int __tipc_sendmsg(struct socket *sock, struct msghdr *m, size_t dlen)
 
 	if (unlikely(syn && !rc)) {
 		tipc_set_sk_state(sk, TIPC_CONNECTING);
-		if (timeout) {
+		if (dlen && timeout) {
 			timeout = msecs_to_jiffies(timeout);
 			tipc_wait_for_connect(sock, &timeout);
 		}