io_uring/rsrc: Annotate struct io_mapped_ubuf with __counted_by

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 checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct io_mapped_ubuf.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: io-uring@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230817212146.never.853-kees@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/rsrc.h

@@ -54,7 +54,7 @@ struct io_mapped_ubuf {
 	u64		ubuf_end;
 	unsigned int	nr_bvecs;
 	unsigned long	acct_pages;
-	struct bio_vec	bvec[];
+	struct bio_vec	bvec[] __counted_by(nr_bvecs);
 };
 
 void io_rsrc_node_ref_zero(struct io_rsrc_node *node);