RDMA/nes: Fix for crash when TX checksum offload is off

When TX checksum offload is disabled for an iWarp connection,
skb->ip_summed needs to be set to CHECKSUM_NONE.

Signed-off-by: Tatyana Nikolova <Tatyana.E.Nikolova@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>