rtla: Set stop threshold after all instances are enabled

rtla: Set stop threshold after all instances are enabled

This avoids startup races where one of the instances hit a threshold
before all instances were enabled, and thus tracing stops without
the relevant event.  In particular, this is not uncommon with the
tests that set a very tight threshold and then complain if there's
no analysis.

This also ensures that we don't stop tracing during a warmup.

The downside is a small chance of having an event over the threshold
early in the output, without stopping on it, which could cause user
confusion.  This should be less likely if the warmup feature is used, but
that doesn't eliminate the race window, just the odds of an unusual spike
right at that moment.

Signed-off-by: Crystal Wood <crwood@redhat.com>
Link: https://lore.kernel.org/r/20251112152529.956778-6-crwood@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>