tty: Preset lock subclass for nested tty locks

Eliminate the requirement of specifying the tty lock nesting at
lock time; instead, set the lock subclass for slave ptys at pty
install (normal ttys and master ptys use subclass 0).

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index bdb8fd1..11db7dc 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -399,6 +399,8 @@
 	if (!o_tty)
 		goto err_put_module;
 
+	tty_set_lock_subclass(o_tty);
+
 	if (legacy) {
 		/* We always use new tty termios data so we can do this
 		   the easy way .. */