Currently open vswitch stores some redundant information in dp_packet like csum_start and csum_offset. These were added early on in the checksum offload work, but we haven't made active use of it to accelerate packet transmission. Furthermore, this information is redundant to the various offsets and protocol flags that we already maintain.

Removing csum_start / csum_offset we can simplify miniflow_extract(), possibly shrink dp_packet, and unify how various netdev types are handled.