For non-TSO offloading, we don't need to access mbuf to know which csum offloading is requested, we can just use the CSUM_{IP,TCP,UDP} in the csum_flags.

For TSO offloading, we still can depend on CSUM_{TSO4,TSO6} in the csum_flags to tell whether the TSO packet is an IPv4 TSO packet or an IPv6 TSO packet.

This streamlines csum offloading handling (remove the two goto) and allows us the nuke the unnecessary get_transport_proto_type().