mds: include nfiles/nsubdirs of directory inode in MClientCaps

mds: include nfiles/nsubdirs of directory inode in MClientCaps

Directory inode's dirstat gets updated by request reply, but not by
cap message. This causes problem for 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.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Fixes: http://tracker.ceph.com/issues/23855
(cherry picked from commit ee2c628f6783954e9b25fab8ac9b572a58666a91)

Conflicts:
	src/messages/MClientCaps.h: Resolved in encode_payload