mds: use OMAP to store dirfrags

MDS can fetch dirfrags from both TMAP and OMAP. When committing a
dirfrags that is stored in TMAP, MDS first uses OSD_OP_TMAP2OMAP
to convert corresponding TMAP to OMAP, then updates the resulting
OMAP.

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