nfs: Annotate struct pnfs_commit_array with __counted_by()

Add the __counted_by compiler attribute to the flexible array member
buckets to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Compile-tested only.

Signed-off-by: Thorsten Blum <[email protected]>
Signed-off-by: Trond Myklebust <[email protected]>

Author: toblux

include/linux/nfs_xdr.h

@@ -1336,7 +1336,7 @@ struct pnfs_commit_array {
 	struct rcu_head rcu;
 	refcount_t refcount;
 	unsigned int nbuckets;
-	struct pnfs_commit_bucket buckets[];
+	struct pnfs_commit_bucket buckets[] __counted_by(nbuckets);
 };
 
 struct pnfs_ds_commit_info {