ARM64: smp: Fix cpu_up() racing with sys_reboot

Nothing stops a process from hotplugging in a CPU concurrently
with a sys_reboot() call. In such a situation we could have
ipi_cpu_stop() mark a cpu as 'offline' and _cpu_up() ignore the
fact that the CPU is not really offline and call the
CPU_UP_PREPARE notifier. When this happens stop_machine code will
complain that the cpu thread already exists and BUG_ON().

CPU0                      CPU1

 sys_reboot()
 kernel_restart()
 machine_restart()
 machine_shutdown()
 smp_send_stop()
 ...                   ipi_cpu_stop()
		       set_cpu_online(1, false)
                       local_irq_disable()
                       while(1)
        <PREEMPT>
 cpu_up()
 _cpu_up()
 if (!cpu_online(1))
 __cpu_notify(CPU_UP_PREPARE...)

 cpu_stop_cpu_callback()
 BUG_ON(stopper->thread)

This is easily reproducible by hotplugging in and out in a tight
loop while also rebooting.

Since the CPU is not really offline and hasn't gone through the
proper steps to be marked as such, let's mark the CPU as inactive.
This is just as easily testable as online and avoids any possibility
of _cpu_up() trying to bring the CPU back online when it never was
offline to begin with. Based on the similar patchset by for arm
targets 040c163( "ARM: smp: Fix cpu_up() racing with sys_reboot)"

Change-Id: Ia13a3aad8cd6616119a07b5114350591173a5d03
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>
Signed-off-by: Venkata Narendra Kumar Gutta <vnkgutta@codeaurora.org>