cxl/pmem: Fix cxl_pmem_region and cxl_memdev leak

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

cxl/pmem: Fix cxl_pmem_region and cxl_memdev leak

When a cxl_nvdimm object goes through a ->remove() event (device

physically removed, nvdimm-bridge disabled, or nvdimm device disabled),

then any associated regions must also be disabled. As highlighted by the

cxl-create-region.sh test [1], a single device may host multiple

regions, but the driver was only tracking one region at a time. This

leads to a situation where only the last enabled region per nvdimm

device is cleaned up properly. Other regions are leaked, and this also

causes cxl_memdev reference leaks.

Fix the tracking by allowing cxl_nvdimm objects to track multiple region

associations.