pppd: De-duplicate Linux rtnetlink code

[pali]

Move common code for sending rtnetlink message into the new send_rtnetlink_msg() function.

[pali]

@jkroonza I added second commit into this PR which de-duplicate rtnetlink stats code. Could you please at it?

[pali]

In principle this looks good and I'm in agreement. I've only looked over the code, and I've not actually tested this right now, happy to assist with that if needed, but I don't have a test environment for this set up currently but can do so if needed.

I have tested second commit just by extracting whole rtnetlink code into separate file / executable and run it and checked that returned values are same as statistics reported by ip -s link output.

[jkroonza]

In principle this looks good and I'm in agreement. I've only looked over the code, and I've not actually tested this right now, happy to assist with that if needed, but I don't have a test environment for this set up currently but can do so if needed.

I have tested second commit just by extracting whole rtnetlink code into separate file / executable and run it and checked that returned values are same as statistics reported by ip -s link output.

Good enough for me. Thank you for your work on this. I'm also quite happy with your other explanations, so from my side: lgtm.