trace/hwlat: Implement the mode config option

Provides the "mode" config to the hardware latency detector. hwlatd has
two different operation modes. The default mode is the "round-robin" one,
in which a single hwlatd thread runs, migrating among the allowed CPUs in a
"round-robin" fashion. This is the current behavior.

The "none" sets the allowed cpumask for a single hwlatd thread at the
startup, but skips the round-robin, letting the scheduler handle the
migration.

In preparation to the per-cpu mode.

Link: https://lkml.kernel.org/r/f3b1271262aa030c680e26615c1b9b2d71e55e92.1624372313.git.bristot@redhat.com

Cc: Phil Auld <pauld@redhat.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Kate Carcia <kcarcia@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Alexandre Chartre <alexandre.chartre@oracle.com>
Cc: Clark Willaims <williams@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>