admission: lock work queue before reading waiting length #131109

When replicated work is submitted for admission, it returns early and admission proceeds asynchronously to the caller. When `V(1)` is enabled, we also log the current queue length in this code path, which is protected by a mutex that wasn't acquired previously. Acquire the queue mutex when `V(1)` is enabled to prevent a race. Part of: cockroachdb#130187 Release note: None