MFC r343582,r343588:Relax BIO_FLUSH ordering in da(4), respecting BIO_ORDERED.

r212160 tightened this from always using MSG_SIMPLE_Q_TAG to always
MSG_ORDERED_Q_TAG. Since it also marked all BIO_FLUSH requests with
BIO_ORDERED, this commit changes nothing immediately, but it returns
BIO_FLUSH callers ability to actually specify ordering they really
need, alike to other request types.