T3655: Fix NAT problem with VRF

Linux netfilter patch https://patchwork.ozlabs.org/project/netfilter-devel/patch/d0f84a97f9c86bec4d537536a26d0150873e640d.1439559328.git.daniel@iogearbox.net/
adds direction support for conntrack zones, which makes it possible to
do NAT with conflicting IP address/port tuples from multiple, isolated tenants on a host.

According to the description of the kernel patch:
>  ... overlapping tuples can be made unique with the zone identifier in
original direction, where the NAT engine will then allocate a unique
tuple in the commonly shared default zone for the reply direction.

I did some basic tests in my lab and it worked fine to forward packets
from eth0 to pppoe0.
- eth0 192.168.1.1/24 in VRF red
- pppoe0 dynamic public IP from ISP VRF default
- set vrf name red protocols static route 0.0.0.0/0 interface pppoe0 vrf 'default'
- set protocols static route 192.168.1.0/24 interface eth0 vrf 'red'

`conntrack -L` shows something like:
```
tcp      6 113 ESTABLISHED src=192.168.1.2 dst=1.1.1.1 sport=58946 dport=80 zone-orig=250 packets=6 bytes=391 src=1.1.1.1 dst=<my-public-ip> sport=80 dport=58946 packets=4 bytes=602 [ASSURED] mark=0 helper=tns use=1
```

It would be much appreciated if someone could test this with more
complex VRF setup.

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