vt: fix unicode console freeing with a common interface

By directly using kfree() in different places we risk missing one if
it is switched to using vfree(), especially if the corresponding
vmalloc() is hidden away within a common abstraction.

Oh wait, that's exactly what happened here.

So let's fix this by creating a common abstraction for the free case
as well.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Reported-by: syzbot+0bfda3ade1ee9288a1be@syzkaller.appspotmail.com
Fixes: 9a98e7a ("vt: don't use kmalloc() for the unicode screen buffer")
Cc: <stable@vger.kernel.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://lore.kernel.org/r/nycvar.YSQ.7.76.2005021043110.2671@knanqh.ubzr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vt.c

@@ -365,9 +365,14 @@ static struct uni_screen *vc_uniscr_alloc(unsigned int cols, unsigned int rows)
 	return uniscr;
 }
 
+static void vc_uniscr_free(struct uni_screen *uniscr)
+{
+	vfree(uniscr);
+}
+
 static void vc_uniscr_set(struct vc_data *vc, struct uni_screen *new_uniscr)
 {
-	vfree(vc->vc_uni_screen);
+	vc_uniscr_free(vc->vc_uni_screen);
 	vc->vc_uni_screen = new_uniscr;
 }
 
@@ -1230,7 +1235,7 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc,
 	err = resize_screen(vc, new_cols, new_rows, user);
 	if (err) {
 		kfree(newscreen);
-		kfree(new_uniscr);
+		vc_uniscr_free(new_uniscr);
 		return err;
 	}