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>

fs/ceph/dir.c

@@ -1143,7 +1143,7 @@ static void ceph_d_prune(struct dentry *dentry)
 {
 	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))

fs/ceph/super.c

@@ -638,10 +638,12 @@ static struct dentry *open_root_dentry(struct ceph_fs_client *fsc,
 	if (err == 0) {
 		dout("open_root_inode success\n");
 		if (ceph_ino(req->r_target_inode) == CEPH_INO_ROOT &&
-		    fsc->sb->s_root == NULL)
+		    fsc->sb->s_root == NULL) {
 			root = d_alloc_root(req->r_target_inode);
-		else
+			ceph_init_dentry(root);
+		} else {
 			root = d_obtain_alias(req->r_target_inode);
+		}
 		req->r_target_inode = NULL;
 		dout("open_root_inode success, root dentry is %p\n", root);
 	} else {