Move CheckBlock() call to critical section
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

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