Make jool client backward compatible (with old Jool kernel modules)

[boleifu]

Please support some levels of backward compatibility between Jool client and the kernel module. Otherwise, we have to reboot the machine or reload the module for every Jool upgrade, which may not be necessary. Thanks.

Error: The kernel module returned error 22: Version mismatch. The userspace client's version is 4.1.4.0,\nbut the kernel module is 4.1.2.0.\nPlease update the kernel module.\n

[ydahhrk]

Ok. ~But since Jool 4.2 is almost complete, I'd like to make this a 4.2 feature.~ ~Also, I don't want to mix 4.1 and 4.2, so backwards compatibility will only extend back up to 4.2.0.~

[ydahhrk]

(Context: Jool 4.2.0's release has taken so long, in the end I decided to patch this bug already. Jool 4.1.9 should be backwards compatible with other 4.1.x's.)

Uhhh... crap.

I just noticed that the kernel module and userspace clients have separate version validation codes. When patching this, I tweaked the kernelspace validation, but not the userspace validation.

So... the patch shouldn't work. However I test it though, it runs fine:

$ jool --version # userspace version
4.1.8.0
$ sudo modprobe jool
$ dmesg | tail -1 # kernelspace version
NAT64 Jool v4.1.9.0 module inserted.
$ sudo jool instance add --iptables -6 64:ff9b::/96
$ echo $?
0

I'm going to have to postpone this bug's closure until 4.1.10, because it needs a review, and 4.1.9 is already released.

[ydahhrk]

Ok, review done.

The kernel module copies the version number from the request to the response, so the client is actually looking at its own version, not the kernel module's. It's one benign bug and one actual bug, cancelling each other out.

So, happily, the backwards compatibility is working as intended in the 4.1.9 release. I'm going to patch the userspace version validation code shortly, but it's not going to change anything in practice.