mds: move cache_trim_throttle perf counter from Server to MDCache

The l_mdss_cache_trim_throttle counter was being accessed from
MDCache::trim_lru() via mds->server->logger, which required a null-pointer
guard and a friend declaration. Since this counter tracks cache trim
throttling, it logically belongs in the MDCache perf counter group.
Move it there and access it via MDCache's own logger directly.

Signed-off-by: Xiubo Li <xiubo.li@clyso.com>