lib: mul_u64_u64_div_u64(): rename parameter 'c' to 'd'

lib: mul_u64_u64_div_u64(): rename parameter 'c' to 'd'

Patch series "Implement mul_u64_u64_div_u64_roundup()", v5.

The pwm-stm32.c code wants a 'rounding up' version of
mul_u64_u64_div_u64().  This can be done simply by adding 'divisor - 1' to
the 128bit product.  Implement mul_u64_add_u64_div_u64(a, b, c, d) = (a *
b + c)/d based on the existing code.  Define mul_u64_u64_div_u64(a, b, d)
as mul_u64_add_u64_div_u64(a, b, 0, d) and mul_u64_u64_div_u64_roundup(a,
b, d) as mul_u64_add_u64_div_u64(a, b, d-1, d).

Only x86-64 has an optimsed (asm) version of the function.  That is
optimised to avoid the 'add c' when c is known to be zero.  In all other
cases the extra code will be noise compared to the software divide code.

The test module has been updated to test mul_u64_u64_div_u64_roundup() and
also enhanced it to verify the C division code on x86-64 and the 32bit
division code on 64bit.

This patch (of 9):

Change to prototype from mul_u64_u64_div_u64(u64 a, u64 b, u64 c) to
mul_u64_u64_div_u64(u64 a, u64 b, u64 d).  Using 'd' for 'divisor' makes
more sense.

An upcoming change adds a 'c' parameter to calculate (a * b + c)/d.

Link: https://lkml.kernel.org/r/20251105201035.64043-1-david.laight.linux@gmail.com
Link: https://lkml.kernel.org/r/20251105201035.64043-2-david.laight.linux@gmail.com
Signed-off-by: David Laight <david.laight.linux@gmail.com>
Reviewed-by: Nicolas Pitre <npitre@baylibre.com>
Cc: Biju Das <biju.das.jz@bp.renesas.com>
Cc: Borislav Betkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Li RongQing <lirongqing@baidu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Cc: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>