ofproto-dpif-rid: Make lookups cheaper.

ofproto-dpif-rid: Make lookups cheaper.

This patch removes a large-ish copy from the recirculation context
lookup, which is performed for each recirculated upcall and
revalidation of a recirculating flow.

Tunnel metadata has grown large since the addition of Geneve options,
and copying that metadata for performing a lookup is not necessary.
Change recirc_metadata to use a pointer to struct flow_tnl, and only
copy the tunnel metadata when needed, and only copy as little of it as
possible.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>