Like __xfrm_transport/mode_tunnel_prep(), this patch is to add
__xfrm_mode_beet_prep() to fix the transport_header for gso
segments, and reset skb mac_len, and pull skb data to the
proto inside esp.

This patch also fixes a panic, reported by ltp:

  # modprobe esp4_offload
  # runltp -f net_stress.ipsec_tcp

  [ 2452.780511] kernel BUG at net/core/skbuff.c:109!
  [ 2452.799851] Call Trace:
  [ 2452.800298]  <IRQ>
  [ 2452.800705]  skb_push.cold.98+0x14/0x20
  [ 2452.801396]  esp_xmit+0x17b/0x270 [esp4_offload]
  [ 2452.802799]  validate_xmit_xfrm+0x22f/0x2e0
  [ 2452.804285]  __dev_queue_xmit+0x589/0x910
  [ 2452.806264]  __neigh_update+0x3d7/0xa50
  [ 2452.806958]  arp_process+0x259/0x810
  [ 2452.807589]  arp_rcv+0x18a/0x1c

It was caused by the skb going to esp_xmit with a wrong transport
header.
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>