suspend-to-idle: Prevent RCU from complaining about tick_freeze()

Put tick_freeze() under RCU_NONIDLE() to prevent RCU from complaining
about suspicious RCU usage in idle by trace_suspend_resume() called
from there.

While at it, fix a comment related to another usage of RCU_NONIDLE()
in enter_freeze_proper().

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>