drbd: Annotate struct fifo_buffer 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 fifo_buffer.

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

Cc: Philipp Reisner <[email protected]>
Cc: Lars Ellenberg <[email protected]>
Cc: Christoph Böhmwalder <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: [email protected]
Cc: [email protected]
Reviewed-by: "Gustavo A. R. Silva" <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

Author: kees

drivers/block/drbd/drbd_int.h

@@ -553,7 +553,7 @@ struct fifo_buffer {
 	unsigned int head_index;
 	unsigned int size;
 	int total; /* sum of all values */
-	int values[];
+	int values[] __counted_by(size);
 };
 extern struct fifo_buffer *fifo_alloc(unsigned int fifo_size);