cxl: Adjust offset calculation for poison injection

cxl: Adjust offset calculation for poison injection

The HPA to DPA translation for poison injection assumes that the
base address starts from where the CXL region begins. When the
extended linear cache is active, the offset can be within the DRAM
region. Adjust the offset so that it correctly reflects the offset
within the CXL region.

[ dj: Add fixes tag from Alison ]

Fixes: c3dd67681c70 ("cxl/region: Add inject and clear poison by region offset")
Link: https://patch.msgid.link/20251031173224.3537030-5-dave.jiang@intel.com
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>