mds: fix root and mdsdir inodes' rsubdirs

inode rstat accounts inode itself.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
(cherry picked from commit da17394941386dab88ddbfed4af2c8cb6b5eb72f)
Merge pull request #2479 from ceph/wip-9444

mds: fix root and mdsdir inodes' rsubdirs

Reviewed-by: Sage Weil <sage@redhat.com>