xfrm: do pskb_pull properly in __xfrm_transport_prep

xfrm: do pskb_pull properly in __xfrm_transport_prep

For transport mode, when ipv6 nexthdr is set, the packet format might
be like:

    ----------------------------------------------------
    |        | dest |     |     |      |  ESP    | ESP |
    | IP6 hdr| opts.| ESP | TCP | Data | Trailer | ICV |
    ----------------------------------------------------

and in __xfrm_transport_prep():

  pskb_pull(skb, skb->mac_len + sizeof(ip6hdr) + x->props.header_len);

it will pull the data pointer to the wrong position, as it missed the
nexthdrs/dest opts.

This patch is to fix it by using:

  pskb_pull(skb, skb_transport_offset(skb) + x->props.header_len);

as we can be sure transport_header points to ESP header at that moment.

It also fixes a panic when packets with ipv6 nexthdr are sent over
esp6 transport mode:

  [  100.473845] kernel BUG at net/core/skbuff.c:4325!
  [  100.478517] RIP: 0010:__skb_to_sgvec+0x252/0x260
  [  100.494355] Call Trace:
  [  100.494829]  skb_to_sgvec+0x11/0x40
  [  100.495492]  esp6_output_tail+0x12e/0x550 [esp6]
  [  100.496358]  esp6_xmit+0x1d5/0x260 [esp6_offload]
  [  100.498029]  validate_xmit_xfrm+0x22f/0x2e0
  [  100.499604]  __dev_queue_xmit+0x589/0x910
  [  100.502928]  ip6_finish_output2+0x2a5/0x5a0
  [  100.503718]  ip6_output+0x6c/0x120
  [  100.505198]  xfrm_output_resume+0x4bf/0x530
  [  100.508683]  xfrm6_output+0x3a/0xc0
  [  100.513446]  inet6_csk_xmit+0xa1/0xf0
  [  100.517335]  tcp_sendmsg+0x27/0x40
  [  100.517977]  sock_sendmsg+0x3e/0x60
  [  100.518648]  __sys_sendto+0xee/0x160

Fixes: c35fe4106b92 ("xfrm: Add mode handlers for IPsec on layer 2")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>