rgw_file: ensure valid_s3_object_name for directories
The logic in RGWLibFS::mkdir() validated bucket names, but not
object names (though RGWLibFS::create() did so).
The negative side effect of this was not creating illegal objects
(we won't), but in a) failing with -EIO and b) more importantly,
not removing up the proposed object from FHCache, which produced a
boost assert when recycled.