bcachefs: Annotate struct bucket_array with __counted_by()

Add the __counted_by compiler attribute to the flexible array member
bucket to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/buckets_types.h

@@ -24,7 +24,7 @@ struct bucket_array {
 	u16			first_bucket;
 	size_t			nbuckets;
 	size_t			nbuckets_minus_first;
-	struct bucket		b[];
+	struct bucket		b[] __counted_by(nbuckets);
 };
 
 struct bucket_gens {