ceph: handle the new nfiles/nsubdirs fields in cap message

Without these new fields, stale st_size is returned in following
case.

1. MDS modifies a directory
2. MDS issues CEPH_CAP_ANY_SHARED to client
3. The client satifies stat(2) by its cached metadata. set st_size
   to "i_files + i_subdirs".

Link: http://tracker.ceph.com/issues/23855


Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>