CVE-2026-45876 arm64/gcs: Fix error handling in arch_set_shadow_stack_status()

In the Linux kernel, the following vulnerability has been resolved: arm64/gcs: Fix error handling in archsetshadowstackstatus allocgcs returns an error-encoded pointer on failure, which comes from dommap, not NULL. The current NULL check fails to detect errors, which could lead to using an invali...