tty: fix logic change introduced by wait_event_interruptible_timeout()

Commit 5a52bd4a2d introduced a subtle logic
change in tty_wait_until_sent().  The original version would only error out
of the 'do { ...  } while (timeout)' loop if signal_pending() evaluated to
true; a timeout or break due to an empty buffer would fall out of the loop
and into the tty->driver->wait_until_sent handling.  The current
implementation will error out on either a pending signal or an empty
buffer, falling through to the tty->driver->wait_until_sent handling only
on a timeout.

The ->wait_until_sent() will not be reached if the buffer empties before
timeout jiffies have elapsed.  This behavior differs from that prior to commit
5a52bd4a2d.

I turned this up while using a little serial download utility to bootstrap an
ARM-based eval board.  The util worked fine on 2.6.22.x, but consistently
failed on 2.6.23.x.  Once I'd determined that, I narrowed things down with git
bisect, and found the above difference in logic in tty_wait_until_sent() by
inspection.

This change reverts the logic flow in tty_wait_until_sent() to match that
prior to the aforementioned commit.

Signed-off-by: Cory T. Tusar <ctusar@videon-central.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Acked-by: Jiri Slaby <jirislaby@gmail.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/tty_ioctl.c

@@ -62,7 +62,7 @@ void tty_wait_until_sent(struct tty_struct * tty, long timeout)
 	if (!timeout)
 		timeout = MAX_SCHEDULE_TIMEOUT;
 	if (wait_event_interruptible_timeout(tty->write_wait,
-			!tty->driver->chars_in_buffer(tty), timeout))
+			!tty->driver->chars_in_buffer(tty), timeout) < 0)
 		return;
 	if (tty->driver->wait_until_sent)
 		tty->driver->wait_until_sent(tty, timeout);