dax: Track all dax_region allocations under a global resource tree

dax: Track all dax_region allocations under a global resource tree

Introduce a global "DAX Regions" resource root and register each
dax_region->res under it via request_resource(). Release the resource on
dax_region teardown.

By enforcing a single global namespace for dax_region allocations, this
ensures only one of dax_hmem or dax_cxl can successfully register a
dax_region for a given range.

Suggested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20260322195343.206900-7-Smita.KoralahalliChannabasappa@amd.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>