ofproto: Move logic for collecting read-only rules into rule_criteria.

ofproto: Move logic for collecting read-only rules into rule_criteria.

ofproto supports OpenFlow tables that the controller can view but not
modify.  Until now, the logic for this was separately implemented in each
flow table operation, which meant that it was somewhat error-prone and in
fact seems missing entirely for "delete" operations.  This commit
implements the "modify" and "delete" side of that logic in the common code
for collecting a group of rules to work on, thereby slightly simplifying
the somewhat tricky modify_flows__() function and fixing the bug in
deletion.

There isn't a good way to reuse this logic for "add" operations.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Thomas Graf <tgraf@suug.ch>