Move CheckBlock() call to critical section
Summary:
This prevents data race for CBlock::fChecked.
Backport of core PR14841
https://github.com/bitcoin/bitcoin/pull/14841/files
Test Plan:
With TSAN enabled:
./src/test/test_bitcoin -t validation_block_tests
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3607