ofp-util: Make make_flow_mod() take cls_rule instead of struct flow.

ofp-util: Make make_flow_mod() take cls_rule instead of struct flow.

This reduces code duplication, by eliminating a function that translates
from "struct flow" to "struct ofp_match" in favor of the existing function
ofputil_cls_rule_to_match().  It also allows the caller to specify the
desired priority (as part of the cls_rule).