powerpc/pseries: Mark accumulate_stolen_time() as notrace

accumulate_stolen_time() is called prior to interrupt state being
reconciled, which can trip the warning in arch_local_irq_restore():

  WARNING: CPU: 5 PID: 1017 at arch/powerpc/kernel/irq.c:258 .arch_local_irq_restore+0x9c/0x130
  ...
  NIP .arch_local_irq_restore+0x9c/0x130
  LR  .rb_start_commit+0x38/0x80
  Call Trace:
    .ring_buffer_lock_reserve+0xe4/0x620
    .trace_function+0x44/0x210
    .function_trace_call+0x148/0x170
    .ftrace_ops_no_ops+0x180/0x1d0
    ftrace_call+0x4/0x8
    .accumulate_stolen_time+0x1c/0xb0
    decrementer_common+0x124/0x160

For now just mark it as notrace. We may change the ordering to call it
after interrupt state has been reconciled, but that is a larger
change.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191024055932.27940-1-mpe@ellerman.id.au

arch/powerpc/kernel/time.c

@@ -232,7 +232,7 @@ static u64 scan_dispatch_log(u64 stop_tb)
  * Accumulate stolen time by scanning the dispatch trace log.
  * Called on entry from user mode.
  */
-void accumulate_stolen_time(void)
+void notrace accumulate_stolen_time(void)
 {
 	u64 sst, ust;
 	unsigned long save_irq_soft_mask = irq_soft_mask_return();