video: fbdev/mmp/core: 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 mmp_path {
...
struct mmp_overlay overlays[0];
};

size = sizeof(struct mmp_path) + count * sizeof(struct mmp_overlay);
instance = kzalloc(size, 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, overlays, count), GFP_KERNEL)

Notice that, in this case, variable size is not necessary, hence it
is removed.

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Bartlomiej Zolnierkiewicz <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/20190807161312.GA26835@embeddedor

drivers/video/fbdev/mmp/core.c

···
 struct mmp_path *mmp_register_path(struct mmp_path_info *info)
 {
 	int i;
-	size_t size;
 	struct mmp_path *path = NULL;
 	struct mmp_panel *panel;
 
-	size = sizeof(struct mmp_path)
-		+ sizeof(struct mmp_overlay) * info->overlay_num;
-	path = kzalloc(size, GFP_KERNEL);
+	path = kzalloc(struct_size(path, overlays, info->overlay_num),
+		       GFP_KERNEL);
 	if (!path)
 		return NULL;