timers: Adjust flseep() to reflect reality

fsleep() simply implements the recommendations of the outdated
documentation in "Documentation/timers/timers-howto.rst". This should be a
user friendly interface to choose always the best timeout function
approach:

- udelay() for very short sleep durations shorter than 10 microseconds
- usleep_range() for sleep durations until 20 milliseconds
- msleep() for the others

The actual implementation has several problems:

- It does not take into account that HZ resolution also has an impact on
  granularity of jiffies and has also an impact on the granularity of the
  buckets of timer wheel levels. This means that accuracy for the timeout
  does not have an upper limit. When executing fsleep(20000) on a HZ=100
  system, the possible additional slack will be 50% as the granularity of
  the buckets in the lowest level is 10 milliseconds.

- The upper limit of usleep_range() is twice the requested timeout. When no
  other interrupts occur in this range, the maximum value is used. This
  means that the requested sleep length has then an additional delay of
  100%.

Change the thresholds for the decisions in fsleep() to make sure the
maximum slack which is added to the sleep duration is 25%.

Note: Outdated documentation will be updated in a followup patch.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/all/20241014-devel-anna-maria-b4-timers-flseep-v3-7-dc8b907cb62f@linutronix.de

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