CVE-2026-74272: cxl/region: Resolve region deletion races

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

cxl/region: Resolve region deletion races

Sungwoo noticed that the sysfs trigger to delete a region may try to delete a region multiple times. It also has no exclusion relative to the kernel releasing the region via CXL root device teardown.

Instead of installing new cxl root devres actions per region, use the existing root decoder unregistration event to remove all remaining regions. An xarray of regions replaces a devres list of regions.

This handles 3 separate issues with the old approach:

1/ sysfs users racing to delete the same region: no longer possible now that the regionslock is held over the lookup and deletion.

2/ multiple actions triggering deletion of the same region: solved by erasing regions while holding @regionslock, and only proceeding on successful erasure.

3/ userspace racing devresreleaseall() to trigger the devres not found warning: solved by sysfs unregistration not requiring a release action