net offloading: Convert checksums to use centrally computed features.

In order to compute the features for other offloads (primarily
scatter/gather), we need to first check the ability of the NIC to
offload the checksum for the packet.  Since we have already computed
this, we can directly use the result instead of figuring it out
again.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>