rtla/timerlat: Make user-space threads the default

After ther -u addition, most of the known users are setting it. And
it makes sense, as it adds more information, and inherits the default
setup for the threads - e.g., cgroups configs.

Thus, if the user-space interface is available, enable -u. Otherwise,
use the in-kernel thread.

Add the -k option to allow the user to request kernel-threads.

Link: https://lkml.kernel.org/r/9241d3089de4091b124f780ed832a0e6646cadaa.1713968967.git.bristot@kernel.org

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>

Documentation/tools/rtla/common_timerlat_options.rst

@@ -27,12 +27,16 @@
         *cyclictest* sets this value to *0* by default, use **--dma-latency** *0* to have
         similar results.
 
+**-k**, **--kernel-threads**
+
+        Use timerlat kernel-space threads, in contrast of **-u**.
+
 **-u**, **--user-threads**
 
         Set timerlat to run without a workload, and then dispatches user-space workloads
         to wait on the timerlat_fd. Once the workload is awakes, it goes to sleep again
         adding so the measurement for the kernel-to-user and user-to-kernel to the tracer
-        output.
+        output. **--user-threads** will be used unless the user specify **-k**.
 
 **-U**, **--user-load**