Prevent an O_NDELAY writer from blocking when a tty write is blocked by the tty atomic writer mutex

Without this a tty write could block if a previous blocking tty write was
in progress on the same tty and blocked by a line discipline or hardware
event.  Originally found and reported by Dave Johnson.

Signed-off-by: Alan Cox <alan@redhat.com>
Acked-by: Dave Johnson <djohnson+linux-kernel@sw.starentnetworks.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/n_tty.c

@@ -1538,7 +1538,8 @@ static unsigned int normal_poll(struct tty_struct * tty, struct file * file, pol
 		else
 			tty->minimum_to_wake = 1;
 	}
-	if (tty->driver->chars_in_buffer(tty) < WAKEUP_CHARS &&
+	if (!tty_is_writelocked(tty) &&
+			tty->driver->chars_in_buffer(tty) < WAKEUP_CHARS &&
 			tty->driver->write_room(tty) > 0)
 		mask |= POLLOUT | POLLWRNORM;
 	return mask;