bcachefs: Prevent a btree iter overflow in alloc path

In bch2_bucket_alloc_trans(), we're iterating over buckets - but not
directly with an iterator, since we're iterating over the freespace
btree.

This means that we need to clear iter->path->preserve, otherwise we'll
end up retaining a btree_path for every alloc key we touched - which is
not what we want here.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/alloc_foreground.c

@@ -351,6 +351,7 @@ static struct open_bucket *try_alloc_bucket(struct btree_trans *trans, struct bc
 	if (!ob)
 		iter.path->preserve = false;
 err:
+	set_btree_iter_dontneed(&iter);
 	bch2_trans_iter_exit(trans, &iter);
 	printbuf_exit(&buf);
 	return ob;