qxl: Use struct_size() in kzalloc()

One of the more common cases of allocation size calculations is finding the
size of a structure that has a zero-sized array at the end, along with memory
for some number of elements for that array. For example:

struct foo {
    int stuff;
    void *entry[];
};

instance = kzalloc(sizeof(struct foo) + sizeof(void *) * count, GFP_KERNEL);

Instead of leaving these open-coded and prone to type mistakes, we can now
use the new struct_size() helper:

instance = kzalloc(struct_size(instance, entry, count), GFP_KERNEL);

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190108162152.GA25361@embeddedorSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/qxl/qxl_display.c

@@ -48,8 +48,8 @@ static int qxl_alloc_client_monitors_config(struct qxl_device *qdev,
 	}
 	if (!qdev->client_monitors_config) {
 		qdev->client_monitors_config = kzalloc(
-				sizeof(struct qxl_monitors_config) +
-				sizeof(struct qxl_head) * count, GFP_KERNEL);
+				struct_size(qdev->client_monitors_config,
+				heads, count), GFP_KERNEL);
 		if (!qdev->client_monitors_config)
 			return -ENOMEM;
 	}