sched: Use WRITE_ONCE() for p->on_rq

Since RCU-tasks uses READ_ONCE(p->on_rq), ensure the write-side
matches with WRITE_ONCE().

Signed-off-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/e4896e0b-eacc-45a2-a7a8-de2280a51ecc@paulmck-laptop