ofproto-dpif: Improve code clarity and comments on recirc changes to rule_dpif_lookup()

This patch improves the code readability and comments on the
recirculation related changes to rule_dpif_lookup() base on off-line
discussions with Jarno.  There is no behavior changes.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>