mod_smacks: Fix to use current method of counting acked stanzas
Fixes #1757
These places seem to have been left since e62025f949f9
The logic around expected_h in should_ack() misbehaved, always comparing
with 0 + unacked instead of acked + unacked.
For full information on our dependencies, version requirements, and
where to find them, see https://prosody.im/doc/depends
If you have luarocks available on your platform, install the following:
- luaexpat
- luasocket
- luafilesystem
- luasec