cxl/region: Fix cxl_region leak, cleanup targets at region delete

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

cxl/region: Fix cxl_region leak, cleanup targets at region delete

When a region is deleted any targets that have been previously assigned

to that region hold references to it. Trigger those references to

drop by detaching all targets at unregister_region() time.

Otherwise that region object will leak as userspace has lost the ability

to detach targets once region sysfs is torn down.