bcachefs: Fix snapshot skiplists during snapshot deletion

In snapshot deleion, we have to pick new skiplist nodes for entries that
point to nodes being deleted.

The function that finds a new skiplist node, skipping over entries being
deleted, was incorrect: if n = 0, but the parent node is being deleted,
we also need to skip over that node.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Kent Overstreet

fs/bcachefs/snapshot.c

@@ -1288,6 +1288,9 @@ static inline u32 bch2_snapshot_nth_parent_skip(struct bch_fs *c, u32 id, u32 n,
 						snapshot_id_list *skip)
 {
 	rcu_read_lock();
+	while (snapshot_list_has_id(skip, id))
+		id = __bch2_snapshot_parent(c, id);
+
 	while (n--) {
 		do {
 			id = __bch2_snapshot_parent(c, id);