ceph: initialize root dentry

Set up d_fsdata on the root dentry.  This fixes a NULL pointer dereference
in ceph_d_prune on umount.  It also means we can eventually strip out all
of the conditional checks on d_fsdata because it is now set unconditionally
(prior to setting up the d_ops).

Fix the ceph_d_prune debug print while we're here.

Signed-off-by: Sage Weil <sage@newdream.net>
diff --git a/fs/ceph/dir.c b/fs/ceph/dir.c
index 2abd0df..bca3948 100644
--- a/fs/ceph/dir.c
+++ b/fs/ceph/dir.c
@@ -1143,7 +1143,7 @@
 {
 	struct ceph_dentry_info *di;
 
-	dout("d_release %p\n", dentry);
+	dout("ceph_d_prune %p\n", dentry);
 
 	/* do we have a valid parent? */
 	if (!dentry->d_parent || IS_ROOT(dentry))