timerlat auto-analysis takes note of all IRQs, before or after the
execution of the timerlat thread.

Because we cannot go backward in the trace (we will fix it when
moving to trace-cmd lib?), timerlat aa take note of the last IRQ
execution in the waiting for the IRQ state, and then print it
if it is executed after the expected timer IRQ starting time.

After the thread sample, the timerlat starts recording the next IRQs as
"previous" irq for the next occurrence.

However, if an IRQ happens after the thread measurement but before the
tracing stops, it is classified as a previous IRQ. That is not
wrong, as it can be "previous" for the subsequent activation. What is
wrong is considering it as a potential source for the last activation.

Ignore the IRQ interference that happens after the IRQ starting time for
now. A future improvement for timerlat can be either keeping a list of
previous IRQ execution or using the trace-cmd library. Still, it requires
further investigation - it is a new feature.

Link: https://lore.kernel.org/lkml/a44a3f5c801dcc697bacf7325b65d4a5b0460537.1691162043.git.bristot@kernel.org

Fixes: 27e348b2 ("rtla/timerlat: Add auto-analysis core")
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>