cxl/region: Block region delete during region creation

cxl/region: Block region delete during region creation

Expand the range lock, rename it "regions_lock", to disable region deletion
in the critical period between construct_region() and attach_target(), as
well as the period between device_add() and registering the remove actions.

Otherwise, userspace can confuse the kernel. It can violate the assumption
the region stays registered through the completion of cxl_add_to_region().
It can violate the assumption that devm_add_action_or_reset() is working
with a live 'struct cxl_region'.

It is ok for the region to disappear outside of those windows as that
mirrors device hotplug flows where the proper locks are held.

Fixes: a32320b71f08 ("cxl/region: Add region autodiscovery")
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/20260519210158.1499795-2-djbw@kernel.org
Signed-off-by: Dave Jiang <dave.jiang@intel.com>