drm/i915/guc: Use struct_size() helper in kmalloc()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c:792:23: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220125180726.GA68646@embeddedor

drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c

@@ -784,7 +784,7 @@ static struct ct_incoming_msg *ct_alloc_msg(u32 num_dwords)
 {
 	struct ct_incoming_msg *msg;
 
-	msg = kmalloc(sizeof(*msg) + sizeof(u32) * num_dwords, GFP_ATOMIC);
+	msg = kmalloc(struct_size(msg, msg, num_dwords), GFP_ATOMIC);
 	if (msg)
 		msg->size = num_dwords;
 	return msg;