rtla/timerlat_top: Fix on-threshold actions firing on signal

rtla/timerlat_top: Fix on-threshold actions firing on signal

A bug was reported when rtla-timerlat-top tool performs on-threshold
actions, even though no threshold was hit. This is reproduced even if no
threshold is set at all:

$ rtla timerlat top -q -c 0 --on-threshold shell,command='echo BAD'
BAD
                                     Timer Latency
...

The bug is due to incorrect logic in timerlat_top_bpf_main_loop().
The loop uses timerlat_bpf_wait(), the return values of which are:

- > 0 (number of ringbuffer entries): at least 1 CPU hit threshold
- = 0: time out
- < 0: wait was interrupted by a signal

Commit 3138df6f0cd0 ("rtla/timerlat: Exit top main loop on any non-zero
wait_retval") changed the condition for "threshold hit" from
"wait_reval == 1" (exactly 1 CPU hit threshold) to "wait_retval != 0",
to fix a race where multiple CPUs hit the threshold at the same time.

That also made it incorrectly include a signal (< 0), coming from either
duration expired (SIGALRM) or user interrupt (SIGINT).

Check for wait_retval greater than zero in the if condition to cover all
return values correctly.

Fixes: 3138df6f0cd0 ("rtla/timerlat: Exit top main loop on any non-zero wait_retval")
Reported-by: Attila Fazekas <afazekas@redhat.com>
Reviewed-by: Wander Lairson Costa <wander@redhat.com>
Link: https://lore.kernel.org/r/20260713141047.687877-1-tglozar@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>