With having in mind that softirq processing is happeing in
ksoftirqd/<cpu> context, which is associated with a specific cpu over
the whole dynamic life time of a system, focussing on CPus as the
dissector appears more sensical.

Quite helpful is this alternative angle of view on the softirqs
processing especially for surveilling the effectiveness of net stack
tunings, as this is highly dynamic depending on the actual net stack
configuration, e.g. scaling or steering.
Signed-off-by: Matthias Tafelmeier <matthias.tafelmeier@gmx.net>