cxl/region: Resolve region deletion races

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 regions_lock is held over the lookup and deletion.

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

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

Fixes: 779dd20cfb56 ("cxl/region: Add region creation support")
Reported-by: Sungwoo Kim <iam@sung-woo.kim>
Closes: http://lore.kernel.org/20260427032010.916681-2-iam@sung-woo.kim
Signed-off-by: Dan Williams <djbw@kernel.org>
Reviewed-by: Alejandro Lucero <alucerop@amd.com>
Tested-by: ALejandro Lucero <alucerop@amd.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20260519210158.1499795-3-djbw@kernel.org
Signed-off-by: Dave Jiang <dave.jiang@intel.com>