stop-machine: Add comment for rcu_momentary_eqs()

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 <tglx@linutronix.de>
Link: https://lore.kernel.org/all/87wmeuanti.ffs@tglx/
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>