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.
Thanks for your interest in contributing to the project!
There are many ways to contribute, such as helping improve the
documentation, reporting bugs, spreading the word or testing the latest
development version.
You can find more information on how to contribute at <https://prosody.im/doc/contributing>
See also the HACKERS and README files.