ceph: avoid dereferencing invalid pointer during cached readdir

Readdir cache keeps array of dentry pointers in page cache. If any
dentry in readdir cache gets pruned, ceph_d_prune() disables readdir
cache for later readdir syscall. The problem is that ceph_d_prune()
ignores unhashed dentry. Ideally MDS should have already revoked
CEPH_CAP_FILE_SHARED (which also disables readdir cache) when dentry
gets unhashed. But if it is somehow MDS does not properly revoke
CEPH_CAP_FILE_SHARED and the unhashed dentry gets pruned later,
ceph_d_prune() will not disable readdir cache, later readdir may
reference invalid dentry pointer.

The fix is make ceph_d_prune() do extra check for unhashed dentry.
Disable readdir cache if the unhashed dentry is still referenced
by readdir cache.

Another fix in this patch is handle d_splice_alias(). If a dentry
gets spliced into new parent dentry, treat it as if it was pruned
(call ceph_d_prune() for it).

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