rpmsg: glink: 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 {
	...
	struct intent_pair intents[];
} __packed * msg;

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(*msg) + sizeof(struct intent_pair) * count

with:

struct_size(msg, intents, count)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

drivers/rpmsg/qcom_glink_native.c

@@ -892,7 +892,7 @@ static void qcom_glink_handle_intent(struct qcom_glink *glink,
 		struct intent_pair intents[];
 	} __packed * msg;
 
-	const size_t msglen = sizeof(*msg) + sizeof(struct intent_pair) * count;
+	const size_t msglen = struct_size(msg, intents, count);
 	int ret;
 	int i;
 	unsigned long flags;