stop-machine: Add comment for rcu_momentary_eqs()

Add a comment to explain the purpose of the rcu_momentary_eqs() call
from multi_cpu_stop(), which is to suppress false-positive RCU CPU
stall warnings.

Reported-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/all/87wmeuanti.ffs@tglx/
Signed-off-by: Paul E. McKenney <[email protected]>
Cc: Valentin Schneider <[email protected]>
Cc: Neeraj Upadhyay <[email protected]>
Cc: Frederic Weisbecker <[email protected]>

Author: paulmckrcu

kernel/stop_machine.c

@@ -250,6 +250,7 @@ static int multi_cpu_stop(void *data)
 			 * be detected and reported on their side.
 			 */
 			touch_nmi_watchdog();
+			/* Also suppress RCU CPU stall warnings. */
 			rcu_momentary_eqs();
 		}
 	} while (curstate != MULTI_STOP_EXIT);