drm/msm: 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 msm_gem_submit {
	...
        struct {
		...
        } bos[0];
};

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(*submit) + ((u64)nr_bos * sizeof(submit->bos[0]))

with:

struct_size(submit, bos, nr_bos)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>

drivers/gpu/drm/msm/msm_gem_submit.c

@@ -30,8 +30,8 @@ static struct msm_gem_submit *submit_create(struct drm_device *dev,
 		uint32_t nr_cmds)
 {
 	struct msm_gem_submit *submit;
-	uint64_t sz = sizeof(*submit) + ((u64)nr_bos * sizeof(submit->bos[0])) +
-		((u64)nr_cmds * sizeof(submit->cmd[0]));
+	uint64_t sz = struct_size(submit, bos, nr_bos) +
+				  ((u64)nr_cmds * sizeof(submit->cmd[0]));
 
 	if (sz > SIZE_MAX)
 		return NULL;