rtla/timerlat_aa: Zero thread sum after every sample analysis

The thread thread_thread_sum accounts for thread interference
during a single activation. It was not being zeroed, so it was
accumulating thread interference over all activations.

It was not that visible when timerlat was the highest priority.

Link: https://lore.kernel.org/lkml/97bff55b0141f2d01b47d9450a5672fde147b89a.1691162043.git.bristot@kernel.org

Fixes: 27e348b2

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