mptcp: update rtx timeout only if required.

We must start the retransmission timer only there are
pending data in the rtx queue.
Otherwise we can hit a WARN_ON in mptcp_reset_timer(),
as syzbot demonstrated.

Reported-and-tested-by: syzbot+42aa53dafb66a07e5a24@syzkaller.appspotmail.com
Fixes: d9ca1de ("mptcp: move page frag allocation in mptcp_sendmsg()")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/mptcp/protocol.c

@@ -1261,11 +1261,12 @@ static void mptcp_push_pending(struct sock *sk, unsigned int flags)
 		mptcp_push_release(sk, ssk, &info);
 
 out:
-	/* start the timer, if it's not pending */
-	if (!mptcp_timer_pending(sk))
-		mptcp_reset_timer(sk);
-	if (copied)
+	if (copied) {
+		/* start the timer, if it's not pending */
+		if (!mptcp_timer_pending(sk))
+			mptcp_reset_timer(sk);
 		__mptcp_check_send_data_fin(sk);
+	}
 }
 
 static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)