tty: Simplify tty->count math in tty_reopen()

BugLink: https://bugs.launchpad.net/bugs/1791758

As notted by Jiri, tty_ldisc_reinit() shouldn't rely on tty counter.
Simplify math by increasing the counter after reinit success.

Cc: Jiri Slaby <jslaby@suse.com>
Link: lkml.kernel.org/r/<20180829022353.23568-2-dima@arista.com>
Suggested-by: Jiri Slaby <jslaby@suse.com>
Reviewed-by: Jiri Slaby <jslaby@suse.cz>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit cf62a1a1)
Signed-off-by: Guilherme G. Piccoli <gpiccoli@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/tty/tty_io.c

@@ -1490,16 +1490,13 @@ static int tty_reopen(struct tty_struct *tty)
 	if (retval)
 		return retval;
 
-	tty->count++;
-	if (tty->ldisc)
-		goto out_unlock;
+	if (!tty->ldisc)
+		retval = tty_ldisc_reinit(tty, tty->termios.c_line);
+	tty_ldisc_unlock(tty);
 
-	retval = tty_ldisc_reinit(tty, tty->termios.c_line);
-	if (retval)
-		tty->count--;
+	if (retval == 0)
+		tty->count++;
 
-out_unlock:
-	tty_ldisc_unlock(tty);
 	return retval;
 }