rcu/kvfree: Remove useless monitor_todo flag

monitor_todo is not needed as the work struct already tracks
if work is pending. Just use that to know if work is pending
using schedule_delayed_work() helper.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>