[POWERPC] Depend on ->initialized in calc_steal_time

If CPU_FTR_PURR is not set, we will never set cpu_purr_data->initialized.
Checking via __get_cpu_var on 64 bit avoids one dependent load compared
to cpu_has_feature in the not-present case, and is always required when
it is present.  The code is under CONFIG_VIRT_CPU_ACCOUNTING so 32 bit
will not be affected.
Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/time.c

@@ -329,11 +329,9 @@ void calculate_steal_time(void)
 	s64 stolen;
 	struct cpu_purr_data *pme;
 
-	if (!cpu_has_feature(CPU_FTR_PURR))
-		return;
 	pme = &__get_cpu_var(cpu_purr_data);
 	if (!pme->initialized)
-		return;		/* this can happen in early boot */
+		return;		/* !CPU_FTR_PURR or early in early boot */
 	tb = mftb();
 	purr = mfspr(SPRN_PURR);
 	stolen = (tb - pme->tb) - (purr - pme->purr);