datapath: Make our checksumming more closely match skb_checksum_help().

datapath: Make our checksumming more closely match skb_checksum_help().

Our code that handles checksumming does essentially the same thing
as skb_checksum_help() except it folds the process into copying to
userspace.  This makes the two functions more closely resemble
each other in structure, including adding a couple of BUG() checks.
This should have no functional change but makes comparision easier
when debugging.