bcachefs: Fix btree_and_journal_iter

We had a bug where btree_and_journal_iter would return the same key
twice - after deleting it (perhaps because it was present in both the
btree and the journal?)

This reworks btree_and_journal_iter to track the current position, much
like btree_paths, which makes the logic considerably simpler and more
robust.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>