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>