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>