mds: add dentries in dirfrag to LRU in reverse order

Files in a dirfrag are usually processed in the order of readdir
results. Files at the beginning of are more likely to be used in
the future than files at the last.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: initialize bloom filter according to dirfrag size

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: ignore CDir::check_rstats() when debug_scatterstat is off

It uses lots of CPU when dirfrag is large

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>