espintcp: restore IP CB before handing the packet to xfrm

Xiumei reported a bug with espintcp over IPv6 in transport mode,
because xfrm6_transport_finish expects to find IP6CB data (struct
inet6_skb_cb). Currently, espintcp zeroes the CB, but the relevant
part is actually preserved by previous layers (first set up by tcp,
then strparser only zeroes a small part of tcp_skb_tb), so we can just
relocate it to the start of skb->cb.

Fixes: e27cca96 ("xfrm: add espintcp (RFC 8229)")
Reported-by: Xiumei Mu <xmu@redhat.com>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>