ofproto-dpif-rid: Store tunnel metadata in frozen metadata directly.

ofproto-dpif-rid: Store tunnel metadata in frozen metadata directly.

"recirc_id_node" contains a 'state_metadata_tunnel' member field.  The
"frozen_metadata" structure used by "recird_id_node" had a 'tunnel'
member that always pointed to 'state_metadata_tunnel".  This commit just
stores the tunnel information directly in "frozen_metadata" instead of
accessing it through a pointer.

This makes the code a bit simpler and easier to reason about.

Signed-off-by: Justin Pettit <jpettit@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>