tty: fix up hung_up_tty_read() conversion

In commit "tty: implement read_iter", I left the read_iter conversion of
the hung up tty case alone, because I incorrectly thought it didn't
matter.

Jiri showed me the errors of my ways, and pointed out the problems with
that incomplete conversion.  Fix it all up.
Reported-by: NJiri Slaby <jirislaby@kernel.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: NJiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/CAHk-=wh+-rGsa=xruEWdg_fJViFG8rN9bpLrfLz=_yBYh2tBhA@mail.gmail.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -431,8 +431,7 @@ struct tty_driver *tty_find_polling_driver(char *name, int *line)
 EXPORT_SYMBOL_GPL(tty_find_polling_driver);
 #endif
 
-static ssize_t hung_up_tty_read(struct file *file, char __user *buf,
-				size_t count, loff_t *ppos)
+static ssize_t hung_up_tty_read(struct kiocb *iocb, struct iov_iter *to)
 {
 	return 0;
 }
@@ -504,7 +503,7 @@ static const struct file_operations console_fops = {
 
 static const struct file_operations hung_up_tty_fops = {
 	.llseek		= no_llseek,
-	.read		= hung_up_tty_read,
+	.read_iter	= hung_up_tty_read,
 	.write_iter	= hung_up_tty_write,
 	.poll		= hung_up_tty_poll,
 	.unlocked_ioctl	= hung_up_tty_ioctl,
@@ -940,8 +939,10 @@ static ssize_t tty_read(struct kiocb *iocb, struct iov_iter *to)
 	/* We want to wait for the line discipline to sort out in this
 	   situation */
 	ld = tty_ldisc_ref_wait(tty);
+	if (!ld)
+		return hung_up_tty_read(iocb, to);
 	i = -EIO;
-	if (ld && ld->ops->read)
+	if (ld->ops->read)
 		i = iterate_tty_read(ld, tty, file, to);
 	tty_ldisc_deref(ld);