scripted-diff: net_processing: Use existing chainman

Summary:
```
-BEGIN VERIFY SCRIPT-
sed -i -E \
    -e 's/g_chainman/m_chainman/g' \
    -e 's@([^:])(Chain(state|)Active)@\1::\2@g' \
    -e 's@::Chain(state|)Active\(\)@m_chainman.ActiveChain\1()@g' \
    -- src/net_processing.cpp
arc lint
-END VERIFY SCRIPT-
```

This is a backport of [[bitcoin/bitcoin#21270 | core#21270]] [11/12]
bitcoin/bitcoin@272d993

Depends on D11495

Test Plan: `ninja all check-all`

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11497