tty: remove tty_lock_nested

This changes all remaining users of tty_lock_nested
to be non-recursive, which lets us kill this function.
As a consequence, we won't need to keep the lock count
any more, which allows more simplifications later.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/char/tty_ldisc.c

@@ -450,9 +450,8 @@ static int tty_ldisc_open(struct tty_struct *tty, struct tty_ldisc *ld)
 	if (ld->ops->open) {
 		int ret;
                 /* BTM here locks versus a hangup event */
-		tty_lock_nested(); /* always held here already */
+		WARN_ON(!tty_locked());
 		ret = ld->ops->open(tty);
-		tty_unlock();
 		return ret;
 	}
 	return 0;