ipv4: Improve martian logs

ipv4: Improve martian logs

At the current moment, the logs for martian packets are as follows:
```
martian source {DST} from {SRC}, on dev {DEV}
martian destination {DST} from {SRC}, dev {DEV}
```

These messages feel rather hard to understand in production, especially
the "martian source" one, mostly because it is grammatically ambitious
to parse which part is now the source address and which part is the
destination address.  For example, "{DST}" may there be interpreted as
the actual source address due to following the word "source", thereby
implying the actual source address to be the destination one.

Personally, I discovered this bug while toying around with TUN
interfaces and using them as a tunnel (receiving packets via a TUN
interface and sending them over a TCP stream; receiving packets from a
TCP stream and writing them to a TUN).[^1]

When these IP addresses contained local IPs (i.e. 10.0.0.0/8 in source
and destination), everything worked fine.  However, sending them to a
real routable IP address on the internet led to them being treated as a
martian packet, obviously.  Using a few sysctl(8) and iptables(8)
settings[^2] fixed it, but while debugging I found the log message
starting with "martian source" rather confusing, as I was unsure on
whether the packet that gets dropped was the packet originating from me
or the response from the endpoint, as "martian source <ROUTABLE IP>"
could also be falsely interpreted as the response packet being martian,
due to the word "source" followed by the routable IP address, implying
the source address of that packet is set to this IP, as explained above.
In the end, I had to look into the source code of the kernel on where
this error message gets generated, which is usually an indicator of
there being room for improvement with regard to this error message.

In terms of improvement, this commit changes the error messages for
martian source and martian destination packets as follows:
```
martian source (src={SRC}, dst={DST}, dev={DEV})
martian destination (src={SRC}, dst={DST}, dev={DEV})
```

These new wordings leave pretty much no room for ambiguity as all
parameters are prefixed with a respective key explaining their semantic
meaning.

See also the following thread on LKML.[^3]

[^1]: <https://backreference.org/2010/03/26/tuntap-interface-tutorial>
[^2]: sysctl net.ipv4.ip_forward=1 && \
      iptables -A INPUT -i tun0 -j ACCEPT && \
      iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
[^3]: <https://lore.kernel.org/all/aSd4Xj8rHrh-krjy@4944566b5c925f79/>

Signed-off-by: Clara Engler <cve@cve.cx>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20260101125114.2608-1-cve@cve.cx
Signed-off-by: Jakub Kicinski <kuba@kernel.org>