cxl/region: Fix out-of-bounds access in cxl_cancel_auto_attach()

In the Linux kernel, the following vulnerability has been resolved:

cxl/region: Fix out-of-bounds access in cxl_cancel_auto_attach()

In cxl_cancel_auto_attach(), it assumes cxled->pos is a valid index for
accessing p->targets[]. However, cxled->pos can be set to negative errno
in cxl_region_sort_targets() if cxl_calc_interleave_pos() fails. This
causes the driver to use a negative index to access p->targets[],
resulting in out-of-bounds access.

Fix it by walking p->targets[] instead of using cxled->pos directly.