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/super.c b/fs/ceph/super.c
index a90846f..8dc73a5 100644
--- a/fs/ceph/super.c
+++ b/fs/ceph/super.c
@@ -638,10 +638,12 @@
 	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 {