ofproto-dpif: Hide struct rule_dpif internally.

ofproto-dpif: Hide struct rule_dpif internally.

By hiding struct rule_dpif inside ofproto-dpif, it becomes very clear
which attributes are accessed by multiple threads and need to be
protected by locks.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>