Unclear kernel requirements for ePPing

[simosund]

With the recent PR#47, ePPing requires a very recent kernel patch to run with XDP (which is the default), which I'm guessing very few Linux devices out there currently have. If attempting to run ePPing (in XDP mode) without this patch the verifier will reject the program (with a somewhat confusing error message), and it may be very unclear for a user what the issue is.

So should probably clarify that the issue is due to an unsupported kernel version, by for example adding an error message that includes suggestions to try with a newer kernel (or a kernel with the mentioned patch backported), or possibly to try and run it with the tc hook instead.

Mostly leaving this as a TODO for myself, so I don't forget it when I get back from my vacation.

[sferlin]

Can we try the default (xdp) and fail with a message pointing to the patch and keep running with the tc hook?

Enjoy your vacation!