ceph: fix d_subdirs ordering problem

We misused list_move_tail() to order the dentry in d_subdirs.
This will screw up the d_subdirs order.

This bug can be reliably reproduced by:
1. mount ceph fs.
2. on ceph fs, git clone git://ceph.newdream.net/git/ceph.git
3. Run autogen.sh in ceph directory.
(Note: Errors only occur at the first time you run autogen.sh.)

Signed-off-by: Henry C Chang <henry_c_chang@tcloudcomputing.com>
Signed-off-by: Sage Weil <sage@newdream.net>