cxl/region: Use do_div() for 64-bit modulo operation

cxl/region: Use do_div() for 64-bit modulo operation

div64_u64_rem() was the wrong choice for doing a modulo operation
and it was used incorrectly, causing a kernel oops by passing NULL
as the remainder parameter. Replace it with the do_div() helper
that does the intended math (gran_offset = offset % gran) and is
architecture safe.

This bug appeared during testing of unaligned address translations.
The visibility to userspace would be limited to folks doing poison
injection or clear by HPA on unaligned regions.

Fixes: 78b50b598462 ("cxl/region: Translate HPA to DPA and memdev in unaligned regions")
Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Link: https://patch.msgid.link/20260117044732.567831-1-alison.schofield@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>