The ipv6 xfrm output path is not aware that packets can be
rerouted by NAT to not use IPsec. We crash in this case
because we expect to have a xfrm state at the dst_entry.
This crash happens if the ipv6 layer does IPsec and NAT
or if we have an interfamily IPsec tunnel with ipv4 NAT.

We fix this by checking for a NAT rerouted packet in each
address family and dst_output() to the new destination
in this case.
Reported-by: Martin Pelikan <martin.pelikan@gmail.com>
Tested-by: Martin Pelikan <martin.pelikan@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>