datapath: nsh: add GSO support

datapath: nsh: add GSO support

Upstream commit:
  commit c411ed854584a71b0e86ac3019b60e4789d88086
  Author: Jiri Benc <jbenc@redhat.com>
  Date:   Mon Aug 28 21:43:24 2017 +0200

    nsh: add GSO support

    Add a new nsh/ directory. It currently holds only GSO functions but more
    will come: in particular, code shared by openvswitch and tc to manipulate
    NSH headers.

    For now, assume there's no hardware support for NSH segmentation. We can
    always introduce netdev->nsh_features later.

    Signed-off-by: Jiri Benc <jbenc@redhat.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Yi Yang <yi.y.yang@intel.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>