include/linux/math.h: fix mult_frac() multiple argument evaluation bug

mult_frac() evaluates _all_ arguments multiple times in the body.

Clarify comment while I'm at it.

Link: https://lkml.kernel.org/r/f9f9fdbb-ec8e-4f5e-a998-2a58627a1a43@p183


Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>