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>