tty: Don't protect atomic operation with mutex

test_bit() is already atomic; drop mutex lock/unlock.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -1391,9 +1391,7 @@ static int tty_reopen(struct tty_struct *tty)
 	}
 	tty->count++;
 
-	mutex_lock(&tty->ldisc_mutex);
 	WARN_ON(!test_bit(TTY_LDISC, &tty->flags));
-	mutex_unlock(&tty->ldisc_mutex);
 
 	return 0;
 }