rpmsg: use struct_size over open coded arithmetic

Replace zero-length array with flexible-array member and make use
of the struct_size() helper in kzalloc(). For example:

struct glink_defer_cmd {
	struct list_head node;

	struct glink_msg msg;
	u8 data[];
};

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Minghao Chi (CGEL ZTE) <chi.minghao@zte.com.cn>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220216030720.1839503-1-chi.minghao@zte.com.cn

drivers/rpmsg/qcom_glink_native.c

@@ -792,7 +792,7 @@ static int qcom_glink_rx_defer(struct qcom_glink *glink, size_t extra)
 		return -ENXIO;
 	}
 
-	dcmd = kzalloc(sizeof(*dcmd) + extra, GFP_ATOMIC);
+	dcmd = kzalloc(struct_size(dcmd, data, extra), GFP_ATOMIC);
 	if (!dcmd)
 		return -ENOMEM;