CVE-2026-89570: cxl/mce: Make the MCE notifier per-region

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

cxl/mce: Make the MCE notifier per-region

Flavien Solt reported lifetime issues with the CXL MCE notifier, which can lead to NULL dereferences and use-after-free in the MCE handler. The notifier was registered per memory device and stored in 'struct cxlmemdevstate', even though it only needs the region state (the region's SPA range and its extended linear cache size).

Instead of keeping the memory device and endpoint alive, the correct fix is to move the notifier into 'struct cxlregion' and register it from cxlregionprobe() as it should be a per-region notifier. Setup the registration to only happen for regions that have an extended linear cache as that is the only current usage.

Remove cxlportgetspacachealias() as it is now dead code.

[ dj: Update devwarn() when notifier fails due to kconfig. (Ben) ]