ofproto: Iterate through exact-match rules first during expiration.

ofproto: Iterate through exact-match rules first during expiration.

A wildcarded flow is idle only if all of its subrules have expired because
they were idle, so unless we expire exact-match rules first it is possible
that a wildcarded flow fails to expire as soon as it should.

(The current implementation of classifier_for_each() iterates through
exact-match rules before wildcarded rules, but nothing in the interface
guarantees that.)