xfrm: Provide private skb extensions for segmented and hw offloaded ESP packets

Commit 94579ac3 ("xfrm: Fix double ESP trailer insertion in IPsec
crypto offload.") added a XFRM_XMIT flag to avoid duplicate ESP trailer
insertion on HW offload. This flag is set on the secpath that is shared
amongst segments. This lead to a situation where some segments are
not transformed correctly when segmentation happens at layer 3.

Fix this by using private skb extensions for segmented and hw offloaded
ESP packets.

Fixes: 94579ac3 ("xfrm: Fix double ESP trailer insertion in IPsec crypto offload.")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>