bcachefs: Prevent a btree iter overflow in alloc path

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>