remoteproc: use struct_size() helper

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 fw_rsc_vdev {
	...
        struct fw_rsc_vdev_vring vring[0];
} __packed;

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

So, replace the following form:

sizeof(*rsc) + rsc->num_of_vrings * sizeof(struct fw_rsc_vdev_vring)

with:

struct_size(rsc, vring, rsc->num_of_vrings)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20190830151406.GA23274@embeddedorSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

drivers/remoteproc/remoteproc_core.c

@@ -477,8 +477,8 @@ static int rproc_handle_vdev(struct rproc *rproc, struct fw_rsc_vdev *rsc,
 	char name[16];
 
 	/* make sure resource isn't truncated */
-	if (sizeof(*rsc) + rsc->num_of_vrings * sizeof(struct fw_rsc_vdev_vring)
-			+ rsc->config_len > avail) {
+	if (struct_size(rsc, vring, rsc->num_of_vrings) + rsc->config_len >
+			avail) {
 		dev_err(dev, "vdev rsc is truncated\n");
 		return -EINVAL;
 	}