x86/cpuidle: don't init stats lock more than once

Osstest flight 122363, having hit an NMI watchdog timeout, shows CPU1 at

Xen call trace:
   [<ffff82d08023d3f4>] _spin_lock+0x30/0x57
   [<ffff82d0802d9346>] update_last_cx_stat+0x29/0x42
   [<ffff82d0802d96f3>] cpu_idle.c#acpi_processor_idle+0x2ff/0x596
   [<ffff82d080276713>] domain.c#idle_loop+0xa8/0xc3

and CPU0 at

Xen call trace:
   [<ffff82d08023d173>] on_selected_cpus+0xb7/0xde
   [<ffff82d0802dbe22>] powernow.c#powernow_cpufreq_target+0x110/0x1cb
   [<ffff82d080257973>] __cpufreq_driver_target+0x43/0xa6
   [<ffff82d080256b0d>] cpufreq_governor_dbs+0x324/0x37a
   [<ffff82d080257bf2>] __cpufreq_set_policy+0xfa/0x19d
   [<ffff82d080256044>] cpufreq_add_cpu+0x3a1/0x5df
   [<ffff82d0802dbab4>] cpufreq_cpu_init+0x17/0x1a
   [<ffff82d0802567a8>] set_px_pminfo+0x2b6/0x2f7
   [<ffff82d08029f1bf>] do_platform_op+0xe75/0x1977
   [<ffff82d0803712c5>] pv_hypercall+0x1f4/0x440
   [<ffff82d0803784a5>] lstar_enter+0x115/0x120

That is, Dom0's ACPI processor driver is in the process of uploading Px
and Cx data. Looking at the ticket lock state in CPU1's registers, it is
waiting for ticket 0x0000 to have its turn, while the supposed current
owner's ticket is 0x0001, which is an invalid state (and neither of the
other two CPUs holds the lock anyway). Hence I can only conclude that
cpuidle_init_cpu(1) ran on CPU 0 while some other CPU held the lock (the
unlock then put the lock in the state that CPU1 is observing).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>

xen/arch/x86/acpi/cpu_idle.c

@@ -842,6 +842,9 @@ int cpuidle_init_cpu(unsigned int cpu)
             acpi_power->states[i].idx = i;
 
         acpi_power->cpu = cpu;
+
+        spin_lock_init(&acpi_power->stat_lock);
+
         processor_powers[cpu] = acpi_power;
     }
 
@@ -849,7 +852,6 @@ int cpuidle_init_cpu(unsigned int cpu)
     acpi_power->states[1].type = ACPI_STATE_C1;
     acpi_power->states[1].entry_method = ACPI_CSTATE_EM_HALT;
     acpi_power->safe_state = &acpi_power->states[1];
-    spin_lock_init(&acpi_power->stat_lock);
 
     return 0;
 }