Detect and rewrite a prefix embedded in an ICMPv6 original packet that was
rewritten by a corresponding DNPT/SNPT rule so it will be recognised by
the host that sent the original packet.

Example

Rules in effect on the 1:2:3:4::/64 + 5:6:7:8::/64 side router:
* SNPT src-pfx 1:2:3:4::/64 dst-pfx 5:6:7:8::/64
* DNPT src-pfx 5:6:7:8::/64 dst-pfx 1:2:3:4::/64

No rules on the 9:a:b:c::/64 side.

1. 1:2:3:4::1 sends UDP packet to 9:a:b:c::1
2. Router applies SNPT changing src to 5:6:7:8::ffef::1
3. 9:a:b:c::1 receives packet with (src 5:6:7:8::ffef::1 dst 9:a:b:c::1)
	and replies with ICMPv6 port unreachable to 5:6:7:8::ffef::1,
	including original packet (src 5:6:7:8::ffef::1 dst 9:a:b:c::1)
4. Router forwards ICMPv6 packet with (src 9:a:b:c::1 dst 5:6:7:8::ffef::1)
	including original packet (src 5:6:7:8::ffef::1 dst 9:a:b:c::1)
	and applies DNPT changing dst to 1:2:3:4::1
5. 1:2:3:4::1 receives ICMPv6 packet with (src 9:a:b:c::1 dst 1:2:3:4::1)
	including original packet (src 5:6:7:8::ffef::1 dst 9:a:b:c::1).
	It doesn't recognise the original packet as the src doesn't
	match anything it originally sent

With this change, at step 4, DNPT will also rewrite the original packet
src to 1:2:3:4::1, so at step 5, 1:2:3:4::1 will recognise the ICMPv6
error and provide feedback to the application properly.

Conversely, SNPT will help when ICMPv6 errors are sent from the
translated network.

1. 9:a:b:c::1 sends UDP packet to 5:6:7:8::ffef::1
2. Router applies DNPT changing dst to 1:2:3:4::1
3. 1:2:3:4::1 receives packet with (src 9:a:b:c::1 dst 1:2:3:4::1)
	and replies with ICMPv6 port unreachable to 9:a:b:c::1
	including original packet (src 9:a:b:c::1 dst 1:2:3:4::1)
4. Router forwards ICMPv6 packet with (src 1:2:3:4::1 dst 9:a:b:c::1)
	including original packet (src 9:a:b:c::1 dst 1:2:3:4::1)
	and applies SNPT changing src to 5:6:7:8::ffef::1
5. 9:a:b:c::1 receives ICMPv6 packet with
	(src 5:6:7:8::ffef::1 dst 9:a:b:c::1) including
	original packet (src 9:a:b:c::1 dst 1:2:3:4::1).
	It doesn't recognise the original packet as the dst doesn't
	match anything it already sent

The change to SNPT means the ICMPv6 original packet dst will be
rewritten to 5:6:7:8::ffef::1 in step 4, allowing the error to be
properly recognised in step 5.

Signed-off-by: Michael Zhou <mzhou@cse.unsw.edu.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>