ip_tunnel: Preserve pointer const in ip_tunnel_info_opts

Change ip_tunnel_info_opts( ) from static function to macro to cast return
value and preserve the const-ness of the pointer.

Signed-off-by: Gavin Li <gavinl@nvidia.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>