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>

fs/bcachefs/btree_iter.c

@@ -2505,6 +2505,7 @@ struct bkey_s_c bch2_btree_iter_peek_slot(struct btree_iter *iter)
 			k = bch2_btree_iter_peek_upto(&iter2, end);
 
 			if (k.k && !bkey_err(k)) {
+				swap(iter->key_cache_path, iter2.key_cache_path);
 				iter->k = iter2.k;
 				k.k = &iter->k;
 			}