arm64: smp: Fix hot-unplug tearing by forcing unregistration

arm64: smp: Fix hot-unplug tearing by forcing unregistration

Sashiko review pointed out the following issue[1].

Commit eba4675008a6 ("arm64: arch_register_cpu() variant to check if
an ACPI handle is now available.") introduced architectural safety
blocks inside arch_unregister_cpu(). If a hot-unplug operation is
determined to be a physical hardware removal (where _STA evaluates to
!ACPI_STA_DEVICE_PRESENT), or if firmware evaluation fails, it aborts
the unregistration transaction early to protect unreadied arm64
infrastructure.

However, returning early from arch_unregister_cpu() causes a catastrophic
state tearing because the generic ACPI layer (acpi_processor_post_eject())
unconditionally continues its cleanup flow. This leaves the stale sysfs
device leaked in the memory, deadlocking any subsequent hot-add attempts
on the same CPU.

Fix it by simplifying arch_unregister_cpu() to always proceed with
the unregistration, as a pr_err_once() warning is sufficient to make
it more visible for currently not supported physical CPU removal.
Also remove the redundant NULL check on acpi_handle as it cannot be
NULL when calling arch_unregister_cpu().

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: stable@vger.kernel.org
Link: https://sashiko.dev/#/patchset/20260520022023.126670-1-ruanjinjie@huawei.com
Fixes: eba4675008a6 ("arm64: arch_register_cpu() variant to check if an ACPI handle is now available.")
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>