core: fix double-readlock in validator

[BurtonQin]

Problem

There is a possible deadlock caused by double readlock in fn Validator::new.

bank_forks: Arc<RwLock<BankForks>>

The first readlock is on L705 and the second on L706

solana/core/src/validator.rs

Lines 704 to 707 in fbf7143

	block_commitment_cache.initialize_slots(
	bank_forks.read().unwrap().working_bank().slot(),
	bank_forks.read().unwrap().root(),
	);

For more details on this kind of deadlock, see
https://www.reddit.com/r/rust/comments/urnqz8/different_behaviors_of_recursive_read_locks_in/

Summary of Changes

Call readlock once and reuse it. After the call, drop the guard.

Fixes #

[nikhayes]

Did you use your tool here to find these? https://github.com/BurtonQin/lockbud

[BurtonQin]

Did you use your tool here to find these? https://github.com/BurtonQin/lockbud

Yes.