iommu/virtio: Add __counted_by for struct viommu_request and use struct_size()

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

While there, use struct_size() helper, instead of the open-coded
version, to calculate the size for the allocation of the whole
flexible structure, including of course, the flexible-array member.

This code was found with the help of Coccinelle, and audited and
fixed manually.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/ZSRFW0yDlDo8+at3@workSigned-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/virtio-iommu.c

@@ -85,7 +85,7 @@ struct viommu_request {
 	void				*writeback;
 	unsigned int			write_offset;
 	unsigned int			len;
-	char				buf[];
+	char				buf[] __counted_by(len);
 };
 
 #define VIOMMU_FAULT_RESV_MASK		0xffffff00
@@ -230,7 +230,7 @@ static int __viommu_add_req(struct viommu_dev *viommu, void *buf, size_t len,
 	if (write_offset <= 0)
 		return -EINVAL;
 
-	req = kzalloc(sizeof(*req) + len, GFP_ATOMIC);
+	req = kzalloc(struct_size(req, buf, len), GFP_ATOMIC);
 	if (!req)
 		return -ENOMEM;