common: compute SimpleLRU's size with contents.size() instead of lru.size()

common: compute SimpleLRU's size with contents.size() instead of lru.size()

As libstdc++ earlier than version 5 implement the list::size() as a O(n) operation,
this should be needed to avoid regression of various ceph component's performance.

Signed-off-by: Xuehan Xu <xuxuehan@360.cn>
(cherry picked from commit 7e0a27a5c8b7d12d378de4d700ed7a95af7860c3)