osd/ECExtentCache: Replace manual mutex operations with std::lock_guard

osd/ECExtentCache: Replace manual mutex operations with std::lock_guard

Replace manual mutex.lock() and mutex.unlock() calls with std::lock_guard
for LRU cache access serialization. This improves code safety by ensuring
automatic unlock on scope exit and reduces maintenance burden compared to
manual mutex management.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>