mm: mm_init: use div64_ul() instead of do_div()

mm: mm_init: use div64_ul() instead of do_div()

Fixes Coccinelle/coccicheck warning reported by do_div.cocci.

Compared to do_div(), div64_ul() does not implicitly cast the divisor and
does not unnecessarily calculate the remainder.

There are no functional changes. The benefit is purely a semantic cleanup
that better communicates the intent of the division and resolves the
static analysis warning.

Signed-off-by: Giorgi Tchankvetadze <giorgitchankvetadze1997@gmail.com>
Link: https://patch.msgid.link/20260602-mm-div64-cleanup-v1-1-bf5d67d89d93@gmail.com
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>