net: Add missing cs_vNodes lock

Summary: Backport of core PR18458.

Test Plan:
With Clang as a compiler:

cmake -GNinja .. -DCMAKE_BUILD_TYPE=Debug
ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8443