tty: Drop tty->count on tty_reopen() failure

In case of tty_ldisc_reinit() failure, tty->count should be decremented
back, otherwise we will never release_tty().
Tetsuo reported that it fixes noisy warnings on tty release like:
  pts pts4033: tty_release: tty->count(10529) != (#fd's(7) + #kopen's(0))

Fixes: commit 892d1fa7 ("tty: Destroy ldisc instance on hangup")

Cc: stable@vger.kernel.org # v4.6+
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Reviewed-by: Jiri Slaby <jslaby@suse.cz>
Tested-by: Jiri Slaby <jslaby@suse.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -1255,6 +1255,7 @@ static void tty_driver_remove_tty(struct tty_driver *driver, struct tty_struct *
 static int tty_reopen(struct tty_struct *tty)
 {
 	struct tty_driver *driver = tty->driver;
+	int retval;
 
 	if (driver->type == TTY_DRIVER_TYPE_PTY &&
 	    driver->subtype == PTY_TYPE_MASTER)
@@ -1268,10 +1269,14 @@ static int tty_reopen(struct tty_struct *tty)
 
 	tty->count++;
 
-	if (!tty->ldisc)
-		return tty_ldisc_reinit(tty, tty->termios.c_line);
-
+	if (tty->ldisc)
 		return 0;
+
+	retval = tty_ldisc_reinit(tty, tty->termios.c_line);
+	if (retval)
+		tty->count--;
+
+	return retval;
 }
 
 /**