cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be brought up again.

cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be brought up again.

A CPU will not show up in virtualized environment which includes an
Enclave. The VM splits its resources into a primary VM and a Enclave
VM. While the Enclave is active, the hypervisor will ignore all requests to
bring up a CPU and this CPU will remain in CPU_UP_PREPARE state.

The kernel will wait up to ten seconds for CPU to show up (do_boot_cpu())
and then rollback the hotplug state back to CPUHP_OFFLINE leaving the CPU
state in CPU_UP_PREPARE. The CPU state is set back to CPUHP_TEARDOWN_CPU
during the CPU_POST_DEAD stage.

After the Enclave VM terminates, the primary VM can bring up the CPU
again.

Allow to bring up the CPU if it is in the CPU_UP_PREPARE state.

[bigeasy: Rewrite commit description.]

Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>
Link: https://lore.kernel.org/r/20220209080214.1439408-3-bigeasy@linutronix.de