ceph: update i_files/i_subdirs only when Fs cap is issued

In MDS, file/subdir counts of a directory inode are protected by
filelock. In request reply without Fs cap, nfiles/nsubdirs can be
stale.

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