mds: make both CInode and CDir as entities of scrub

mds: make both CInode and CDir as entities of scrub

Making CDir as entity of scrub is preparetion for scrubbing across
multiple mds. When subtree bound is encountered, scrub should be
forwarded to subtree's auth mds. The auth mds adds CDir to scrub stack.

Signed-off-by: Simon Gao <simon29rock@gmail.com>
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>