xfrm6: Fix CHECKSUM_COMPLETE after IPv6 header push

xfrm6_transport_finish rebuilds the IPv6 header based on the
original one and pushes it back without fixing skb->csum.
Therefore, CHECKSUM_COMPLETE is no longer valid and the packet
gets dropped.

Fix skb->csum by calling skb_postpush_rcsum.

Note: A valid IPv4 header has checksum 0, unlike IPv6. Thus,
the change is not needed in the sibling xfrm4_transport_finish
function.

Signed-off-by: Yossi Kuperman <yossiku@mellanox.com>
Signed-off-by: Ilan Tayari <ilant@mellanox.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>