riscv: Clean up & optimize unaligned scalar access probe

riscv: Clean up & optimize unaligned scalar access probe

check_unaligned_access_speed_all_cpus() is more complicated than it should
be:

  - It uses on_each_cpu() to probe unaligned memory access on all CPUs but
    excludes CPU0 with a check in the callback function. So an IPI to CPU0
    is wasted.

  - Probing on CPU0 is done with smp_call_on_cpu(), which is not as fast as
    on_each_cpu().

The reason for this design is because the probe is timed with jiffies.
Therefore on_each_cpu() excludes CPU0 because that CPU needs to tend to
jiffies.

Instead, replace jiffies usage with ktime_get_mono_fast_ns(). With jiffies
out of the way, on_each_cpu() can be used for all CPUs and
smp_call_on_cpu() can be dropped.

To make ktime_get_mono_fast_ns() usable, move this probe to late_initcall.
Anything after clocksource's fs_initcall works, but avoid depending on
clocksource staying at fs_initcall.

The choice of probe time is now 8000000 ns, which is the same as before (2
jiffies) for riscv defconfig. This is excessive for the CPUs I have, and
probably should be reduced; but that's a different discussion.

Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Nam Cao <namcao@linutronix.de>
Link: https://patch.msgid.link/9b9a20affe2e4f5c380926ceb885a47e20a59395.1770830596.git.namcao@linutronix.de
Signed-off-by: Paul Walmsley <pjw@kernel.org>