ofp-util: Properly handle "tun_id"s in tun_id_from_cookie flows.

Just setting the tun_id field isn't enough--it's also necessary to set
the tun_id_mask.  Otherwise the call to cls_rule_zero_wildcarded_fields()
at the end of ofputil_cls_rule_from_match() will zero out the tun_id again.

This was broken by commit 8368c090cab "Implement arbitrary bitwise masks
for tun_id field" back in January.  (This makes me wonder whether we can
drop support for tun_id_from_cookie now.)

Reported-by: Dan Wendlandt <dan@nicira.com>