ocfs2: Annotate struct ocfs2_slot_info 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 ocfs2_slot_info.

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

Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: ocfs2-devel@lists.linux.dev
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230915201522.never.979-kees@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: kees

fs/ocfs2/slot_map.c

@@ -37,7 +37,7 @@ struct ocfs2_slot_info {
 	unsigned int si_blocks;
 	struct buffer_head **si_bh;
 	unsigned int si_num_slots;
-	struct ocfs2_slot si_slots[];
+	struct ocfs2_slot si_slots[] __counted_by(si_num_slots);
 };