datapath: Add function to copy skb checksum bits.

datapath: Add function to copy skb checksum bits.

Some kernels don't copy the checksum offload state in the skb
header when doing different types of copies.  Xen adds even more
fields, which are also not consistently copied.  The result is
uninitialized memory and random outcomes.  This adds a function to
consistently copy these bits across all kernel versions.