flow: use rule priority in match_format() function

flow: use rule priority in match_format() function

The flow_format() function was incorrectly passing skb_priority
to the match_format() function. match_format() function instead
expects rule priority.

This issue was introduced with aa6c9932f2937fa9a2140ec1737668eb9105b0b5
(Change logging format for flows to that accepted by ofproto/trace).

Signed-off-by: Ansis Atteka <aatteka@nicira.com>