vfs_ceph_snapshots: fix root relative path handling
For file paths relative to root, ceph_snap_get_parent_path() may return an empty parent dir string, in which case the CephFS snashot path should be ".snap".
Bug: https://bugzilla.samba.org/show_bug.cgi?id=14216
Signed-off-by: David Disseldorp ddiss@samba.org