nexthop: Annotate struct nh_notifier_res_table_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 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
nh_notifier_res_table_info.

Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Tom Rix <trix@redhat.com>
Cc: llvm@lists.linux.dev
Link: https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci [1]
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20231003231818.work.883-kees@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/nexthop.h

@@ -200,7 +200,7 @@ struct nh_notifier_res_bucket_info {
 
 struct nh_notifier_res_table_info {
 	u16 num_nh_buckets;
-	struct nh_notifier_single_info nhs[];
+	struct nh_notifier_single_info nhs[] __counted_by(num_nh_buckets);
 };
 
 struct nh_notifier_info {