Fix a -Wshadow warning in net_processing.cpp

Summary:
The peer variable shadows the function argument. The stat variable is fine but very close to stats which can be confusing.
This diff prefixes the local variables with an underscore to make it less confusing and avoid the shadowing.

Test Plan:
With Clang:

ninja

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15198