ceph: stop on-going cached readdir if mds revokes FILE_SHARED cap

If directory's FILE_SHARED cap get revoked, dentry in the directory
can get spliced into other directory (Eg, other client move the
dentry into directory B, then we do readdir on directory B). So we
should stop on-going cached readdir. this can be achieved by marking
dir not complete, because __dcache_readdir() checks dir completeness
before emitting each dentry.

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