scripted-diff: Rename CChainState::g_failed_blocks to m_failed_blocks

Summary:
To reflect its actual status as a member rather than a global value.

g_failed_blocks was previously global: 2862aca

-BEGIN VERIFY SCRIPT-
sed -i 's/g_failed_blocks/m_failed_blocks/g' src/validation.cpp
-END VERIFY SCRIPT-

Backport of Core PR13016
https://github.com/bitcoin/bitcoin/pull/13016/files
Completes T579

Depends on D1972

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3693