ceph: fix ceph_mdsc_build_path to not stop on first component

When ceph_mdsc_build_path is handed a positive dentry, it will return a
zero-length path string with the base set to that dentry.  This is not
what we want.  Always include at least one path component in the string.

ceph_mdsc_build_path has behaved this way for a long time but it didn't
matter until recent d_name handling rework.

Fixes: 964fff74

 ("ceph: use ceph_mdsc_build_path instead of clone_dentry_name")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>