mds: initialize some member variables of MDCache

I added some member variables to class MDCache, but forget to
initialize them.

Fixes: #5236
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: allow purging "dirty parent" stray inode

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