TTY: introduce deinit helpers for proper ldisc shutdown

Introduce deinitialize_tty_struct which should be called after
initialize_tty_struct and before successfull tty_ldisc_setup.

It calls tty_ldisc_deinit which is opposite of tty_ldisc_init. It only
puts a reference to ldisc and assigns NULL to tty->ldisc.

It will be used to shut down ldisc when tty_release cannot be called
yet.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 026bf2f..f5dd235 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2887,6 +2887,20 @@
 }
 
 /**
+ *	deinitialize_tty_struct
+ *	@tty: tty to deinitialize
+ *
+ *	This subroutine deinitializes a tty structure that has been newly
+ *	allocated but tty_release cannot be called on that yet.
+ *
+ *	Locking: none - tty in question must not be exposed at this point
+ */
+void deinitialize_tty_struct(struct tty_struct *tty)
+{
+	tty_ldisc_deinit(tty);
+}
+
+/**
  *	tty_put_char	-	write one character to a tty
  *	@tty: tty
  *	@ch: character