banman: reformulate nBanUtil calculation

Summary:
Avoid reassigning parameters.

This is a partial backport of Core PR14605 : https://github.com/bitcoin/bitcoin/pull/14605/commits/1ffa4ce27d4ea6c1067d8984455df97994c7713e

Depends on D4127

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4128