cpufreq: powernv: Disable preemption while checking CPU throttling state

With preemption turned on we can read incorrect throttling state
while being switched to CPU on a different chip.

 BUG: using smp_processor_id() in preemptible [00000000] code: cat/7343
 caller is .powernv_cpufreq_throttle_check+0x2c/0x710
 CPU: 13 PID: 7343 Comm: cat Not tainted 4.8.0-rc5-dirty #1
 Call Trace:
 [c0000007d25b75b0] [c000000000971378] .dump_stack+0xe4/0x150 (unreliable)
 [c0000007d25b7640] [c0000000005162e4] .check_preemption_disabled+0x134/0x150
 [c0000007d25b76e0] [c0000000007b63ac] .powernv_cpufreq_throttle_check+0x2c/0x710
 [c0000007d25b7790] [c0000000007b6d18] .powernv_cpufreq_target_index+0x288/0x360
 [c0000007d25b7870] [c0000000007acee4] .__cpufreq_driver_target+0x394/0x8c0
 [c0000007d25b7920] [c0000000007b22ac] .cpufreq_set+0x7c/0xd0
 [c0000007d25b79b0] [c0000000007adf50] .store_scaling_setspeed+0x80/0xc0
 [c0000007d25b7a40] [c0000000007ae270] .store+0xa0/0x100
 [c0000007d25b7ae0] [c0000000003566e8] .sysfs_kf_write+0x88/0xb0
 [c0000007d25b7b70] [c0000000003553b8] .kernfs_fop_write+0x178/0x260
 [c0000007d25b7c10] [c0000000002ac3cc] .__vfs_write+0x3c/0x1c0
 [c0000007d25b7cf0] [c0000000002ad584] .vfs_write+0xc4/0x230
 [c0000007d25b7d90] [c0000000002aeef8] .SyS_write+0x58/0x100
 [c0000007d25b7e30] [c00000000000bfec] system_call+0x38/0xfc

Fixes: 09a972d1 (cpufreq: powernv: Report cpu frequency throttling)
Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpufreq/powernv-cpufreq.c

@@ -664,8 +664,14 @@ static int powernv_cpufreq_target_index(struct cpufreq_policy *policy,
 	if (unlikely(rebooting) && new_index != get_nominal_index())
 		return 0;
 
-	if (!throttled)
+	if (!throttled) {
+		/* we don't want to be preempted while
+		 * checking if the CPU frequency has been throttled
+		 */
+		preempt_disable();
 		powernv_cpufreq_throttle_check(NULL);
+		preempt_enable();
+	}
 
 	cur_msec = jiffies_to_msecs(get_jiffies_64());