classifier: Optimize iteration with a catch-all target rule.

When cls_cursor_init() is given a NULL target, it can skip an expensive
step comparing the rule against the target for every table and every rule
in the classifier.  collect_rule_loose() and other callers could take
advantage of this optimization, except that they actually pass in a rule
that matches everything instead of a NULL rule (e.g. for "ovs-ofctl
dump-flows <bridge>" without specifying a matching rule).

This optimizes that case.

Signed-off-by: Ben Pfaff <blp@nicira.com>

lib/classifier.h

@@ -78,6 +78,8 @@ uint32_t cls_rule_hash(const struct cls_rule *, uint32_t basis);
 
 void cls_rule_format(const struct cls_rule *, struct ds *);
 
+bool cls_rule_is_catchall(const struct cls_rule *);
+
 bool cls_rule_is_loose_match(const struct cls_rule *rule,
                              const struct match *criteria);