ceph: disable cached readdir after dropping positive dentry

Ideally CEPH_CAP_FILE_SHARED should have been revoked before
postive dentry get dropped. But if something goes wrong, later
cached readdir may dereference the dropped dentry.

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