ibmveth: fix bad UDP checksums

This patch fixes a ibmveth bug where bad UDP checksums are being transmitted
when checksum offloading is enabled.
The hypervisor does checksum offloading only on TCP packets, so ibmveth calls
skb_checksum_help() for any other protocol.  The bug happens because
the packet is being modified after the DMA map, so we would need a memory
barrier before making the hypervisor call.  Reordering the code so that the
DMA map happens after skb_checksum_help() has the additional advantage of
fixing a DMA map leak if skb_checksum_help() where to fail.

Signed-off-by: Santiago Leon <santil@us.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>