mds: remove mdsdir in the final step of shutdown MDS

Otherwise we may get bad subtree map if we restart the MDS before
the shutdown process finishes.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #1751 from ceph/wip-mds-shutdown

mds: remove mdsdir in the final step of shutdown MDS

Reviewed-by: Sage Weil <sage@inktank.com>