kernel: hung_task.c: disable on suspend

kernel: hung_task.c: disable on suspend

It is possible to observe hung_task complaints when system goes to
suspend-to-idle state:

 # echo freeze > /sys/power/state

 PM: Syncing filesystems ... done.
 Freezing user space processes ... (elapsed 0.001 seconds) done.
 OOM killer disabled.
 Freezing remaining freezable tasks ... (elapsed 0.002 seconds) done.
 sd 0:0:0:0: [sda] Synchronizing SCSI cache
 INFO: task bash:1569 blocked for more than 120 seconds.
       Not tainted 4.19.0-rc3_+ #687
 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
 bash            D    0  1569    604 0x00000000
 Call Trace:
  ? __schedule+0x1fe/0x7e0
  schedule+0x28/0x80
  suspend_devices_and_enter+0x4ac/0x750
  pm_suspend+0x2c0/0x310

Register a PM notifier to disable the detector on suspend and re-enable
back on wakeup.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>