ofproto-dpif: Keep subfacets longer to avoid assert-fail in facet_account().

ofproto-dpif: Keep subfacets longer to avoid assert-fail in facet_account().

If a subfacet expired when its facet still had statistics that had not
yet been pushed into the rule, and the facet either used the "normal"
action or the bridge contained a bond port, then facet_account() would
be called after the last subfacet was removed from its facet's list of
subfacets, triggering an assertion failure in list_front().

This fixes the problem by always running facet_flush_stats() (which calls
facet_account()) before deleting the last subfacet from a facet.

This problem took a while to surface because subfacets usually expire only
long after their statistics have been pushed into the rule.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Reported-by: Mike Kruze <mkruze@nicira.com>
Bug #9074.