timekeeping: Fix references to nonexistent ktime_get_fast_ns()

There was never a function named ktime_get_fast_ns().
Presumably these should refer to ktime_get_mono_fast_ns() instead.

Fixes: c1ce406e

 ("timekeeping: Fix up function documentation for the NMI safe accessors")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/r/06df7b3cbd94f016403bbf6cd2b38e4368e7468f.1682516546.git.geert+renesas@glider.be