nsh: set mac len based on inner packet

When pulling the NSH header in nsh_gso_segment, set the mac length
based on the encapsulated packet type.

skb_reset_mac_len computes an offset to the network header, which
here still points to the outer packet:

  >     skb_reset_network_header(skb);
  >     [...]
  >     __skb_pull(skb, nsh_len);
  >     skb_reset_mac_header(skb);    // now mac hdr starts nsh_len == 8B after net hdr
  >     skb_reset_mac_len(skb);       // mac len = net hdr - mac hdr == (u16) -8 == 65528
  >     [..]
  >     skb_mac_gso_segment(skb, ..)

Link: http://lkml.kernel.org/r/CAF=yD-KeAcTSOn4AxirAxL8m7QAS8GBBe1w09eziYwvPbbUeYA@mail.gmail.com
Reported-by:  <syzbot+7b9ed9872dab8c32305d@syzkaller.appspotmail.com>
Fixes: c411ed85

 ("nsh: add GSO support")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>