To prepare for future IPv6 NAT functions.
- Rename common ipv4_nat_xxx to nat_xxx
- Move some common definitions into header

Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>

Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>

Notes:
1. This version of NAT66 is a stateful one like NAT44, corresponding to Linux's MASQUERADE target of ip6tables.  We can support stateless NAT66 & NPTv6 later by slightly modify the address & port selection logic (maybe just match the rules and skip the entry find).
2. We're using same flag `IFF_NAT` for both NAT44 & NAT66 to make control easier.  Which means, if we enable NAT, both NAT44 & NAT66 will be enabled.  If we don't want one of them, we can just disable that one in Kconfig.
3. Maybe we can accelerate the checksum adjustment by pre-calculate a difference of checksum, and apply it to each packet, instead of calling `net_chksum_adjust` each time.  Just a thought, maybe do it later.
4. IP fragment segments on NAT66 connections are not supported yet.

Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>

And fix possibly dead loop.

Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>