Due to the avoidance of IPIs to idle CPUs arch_freq_get_on_cpu() can return
0 when the last sample was too long ago.

show_cpuinfo() has a fallback to cpufreq_quick_get() and if that fails to
return cpu_khz, but the readout code for the per CPU scaling frequency in
sysfs does not.

Move that fallback into arch_freq_get_on_cpu() so the behaviour is the same
when reading /proc/cpuinfo and /sys/..../cur_scaling_freq.
Suggested-by: "Rafael J. Wysocki" <rafael@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Doug Smythies <dsmythies@telus.net>
Link: https://lore.kernel.org/r/87pml5180p.ffs@tglx