[media] v4l2-ctrls: fix integer overflow in v4l2_g_ext_ctrls()

A large cs->count from userspace may overflow the allocation size,
leading to memory corruption.  v4l2_g_ext_ctrls() can be reached
from subdev_do_ioctl() or __video_do_ioctl().

Use kmalloc_array() to avoid the overflow.
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/v4l2-ctrls.c

@@ -2036,7 +2036,8 @@ int v4l2_g_ext_ctrls(struct v4l2_ctrl_handler *hdl, struct v4l2_ext_controls *cs
 		return class_check(hdl, cs->ctrl_class);
 
 	if (cs->count > ARRAY_SIZE(helper)) {
-		helpers = kmalloc(sizeof(helper[0]) * cs->count, GFP_KERNEL);
+		helpers = kmalloc_array(cs->count, sizeof(helper[0]),
+					GFP_KERNEL);
 		if (helpers == NULL)
 			return -ENOMEM;
 	}