staging: greybus: Add __counted_by for struct apr_rx_buf 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: Kees Cook <keescook@chromium.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Link: https://lore.kernel.org/r/ZSR2O6zGyT/VX6ve@workSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/greybus/raw.c

@@ -29,7 +29,7 @@ struct gb_raw {
 struct raw_data {
 	struct list_head entry;
 	u32 len;
-	u8 data[];
+	u8 data[] __counted_by(len);
 };
 
 static const struct class raw_class = {
@@ -73,7 +73,7 @@ static int receive_data(struct gb_raw *raw, u32 len, u8 *data)
 		goto exit;
 	}
 
-	raw_data = kmalloc(sizeof(*raw_data) + len, GFP_KERNEL);
+	raw_data = kmalloc(struct_size(raw_data, data, len), GFP_KERNEL);
 	if (!raw_data) {
 		retval = -ENOMEM;
 		goto exit;