mlxsw: spectrum_ipip: Split accessor functions

To implement NETDEV_CHANGE notifications on IP-in-IP tunnels, the
handler needs to figure out what actually changed, to understand how
exactly to update the offloads. It will do so by storing struct
ip_tunnel_parm with previous configuration, and comparing that to the
new version.

To facilitate these comparisons, extract the code that operates on
struct ip_tunnel_parm from the existing accessor functions, and make
those a thin wrapper that extracts tunnel parameters and dispatches.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>