xfrm6: Fix transport header offset in _decode_session6.
xfrm6: Fix transport header offset in _decode_session6.

skb->transport_header might not be valid when we do a reverse
decode because the ipv6 tunnel error handlers don't update it
to the inner transport header. This leads to a wrong offset
calculation and to wrong layer 4 informations. We fix this
by using the size of the ipv6 header as the first offset.

Signed-off-by: Steffen Klassert <66deb1d63f85161cc287ce2bac0c65cb5ae958a7@secunet.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs