x86 acpi: Fix crash in enable_nonboot_cpus() on wakeup from S3/S4

Bringing a CPU back online can require RCU work to be flushed, because
the per-cpu data from last time the CPU was online may not yet be
deallocated. Use the new rcu_barrier() interface function to achieve
this.

Signed-off-by: Keir Fraser <keir@xen.org>

xen/arch/x86/acpi/power.c

@@ -206,6 +206,7 @@ static int enter_state(u32 state)
  enable_cpu:
     cpufreq_add_cpu(0);
     microcode_resume_cpu(0);
+    rcu_barrier();
     mtrr_aps_sync_begin();
     enable_nonboot_cpus();
     mtrr_aps_sync_end();