banman: pass the banfile path in

Summary:
There's no need to hard-code the path here. Passing it in means that there are
no ordering concerns wrt establishing the datadir.

This is a partial backport of Core PR14605 : https://github.com/bitcoin/bitcoin/pull/14605/commits/2e56702ecedd83c4b7cb8de9de5c437c8c08e645

Depends on D4114

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4117