nx-match: Encode dp_hash and recirc_id in OXM also.

dp_hash and recirc_id are specific to OVS, but that doesn't mean that we
shouldn't encode them into flow matches when OXM is used in OpenFlow 1.2
and later.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>