tty: fix data race in flush_to_ldisc

flush_to_ldisc reads port->itty and checks that it is not NULL,
concurrently release_tty sets port->itty to NULL. It is possible
that flush_to_ldisc loads port->itty once, ensures that it is
not NULL, but then reloads it again and uses. The second load
can already return NULL, which will cause a crash.

Use READ_ONCE to read port->itty.

The data race was found with KernelThreadSanitizer (KTSAN).
Signed-off-by: NDmitry Vyukov <dvyukov@google.com>
Reviewed-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_buffer.c

@@ -467,7 +467,7 @@ static void flush_to_ldisc(struct work_struct *work)
 	struct tty_struct *tty;
 	struct tty_ldisc *disc;
 
-	tty = port->itty;
+	tty = READ_ONCE(port->itty);
 	if (tty == NULL)
 		return;