soc: qcom: apr: 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: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/ZSRw6RNi3Crhd32H@workSigned-off-by: Bjorn Andersson <andersson@kernel.org>

drivers/soc/qcom/apr.c

@@ -41,7 +41,7 @@ struct packet_router {
 struct apr_rx_buf {
 	struct list_head node;
 	int len;
-	uint8_t buf[];
+	uint8_t buf[] __counted_by(len);
 };
 
 /**
@@ -171,7 +171,7 @@ static int apr_callback(struct rpmsg_device *rpdev, void *buf,
 		return -EINVAL;
 	}
 
-	abuf = kzalloc(sizeof(*abuf) + len, GFP_ATOMIC);
+	abuf = kzalloc(struct_size(abuf, buf, len), GFP_ATOMIC);
 	if (!abuf)
 		return -ENOMEM;