net: ipv6: Fixed up ipsec packet be re-routing issue

Bug report on https://bugzilla.kernel.org/show_bug.cgi?id=75781



When a local output ipsec packet match the mangle table rule,
and be set mark value, the packet will be route again in
route_me_harder -> _session_decoder6

In this case, the nhoff in CB of skb was still the default
value 0. So the protocal match can't success and the packet can't match
correct SA rule,and then the packet be send out in plaintext.

To fixed up the issue. The CB->nhoff must be set.

Signed-off-by: Hui Zhang <huizhang@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>