Fix incorrect current_elements_num in fs cache

Why it matters

The fix addresses problems with inaccurate tracking of the number of current elements in the filesystem cache, ensuring more reliable cache metrics and behavior. This improves the stability and correctness of cache management in ClickHouse.

How to use it

This fix is applied internally within the filesystem cache mechanism. Users benefit from the improved accuracy of current_elements_num without needing to change any configuration or usage patterns.