If we process these in sequence in software interrupt, everything
is fine.  Processing of "A" will add the address, and the test
ping packet "B" will respond properly.

If you defer "A", everything breaks and the test packet "B" will
get processed first and not work.

Playing devil's advocate here: if the packets are processed on
two different CPUs then this could also happen and break the test
case.

So the test is probably a bit fragile.

Currently it is unlikely to happen because of interrupt affinity for a 
single device,  but in future with MSI-X support it might not.

I generally agree it's better to keep this in kernel though.

-Andi