osd: introduce TMAP->OMAP conversion operation

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
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>
osd/OSDMap: get_up_osd_features()

Signed-off-by: Sage Weil <sage@inktank.com>
mds: require CEPH_FEATURE_OSD_TMAP2OMAP

Require that all OSDs support TMAP2OMAP before starting the MDS.  This
avoids doing some work and then crashing with EOPNOTSUPP, and gives us
a more informative message in the logs.

Signed-off-by: Sage Weil <sage@inktank.com>