bcachefs: Save key_cache_path in peek_slot()

When bch2_btree_iter_peek_slot() clones the iterator to search for the
next key, and then discovers that the key from the cloned iterator is
the key we want to return - we also want to save the
iter->key_cache_path as well, for the update path.

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