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>

fs/ceph/inode.c

@@ -827,7 +827,7 @@ static void ceph_set_dentry_offset(struct dentry *dn)
 
 	spin_lock(&dcache_lock);
 	spin_lock(&dn->d_lock);
-	list_move_tail(&dir->d_subdirs, &dn->d_u.d_child);
+	list_move(&dn->d_u.d_child, &dir->d_subdirs);
 	dout("set_dentry_offset %p %lld (%p %p)\n", dn, di->offset,
 	     dn->d_u.d_child.prev, dn->d_u.d_child.next);
 	spin_unlock(&dn->d_lock);