mds: start internal MDS request for fragmentating directory

mds: start internal MDS request for fragmentating directory

Start internal MDS request for fragmentating directory operation. With
MDS request, we can easily acquire locks required by the fragmentating
directory operation. (The old way to get locks is 'try lock' style,
which is not reliable)

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