nohz/full, sched/rt: Fix missed tick-reenabling bug in dequeue_task_rt()

stable inclusion
from stable-v4.19.256
commit 952146183df68a7fb29c3c29d56ffdc941fbdc39
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ
CVE: NA

--------------------------------

[ Upstream commit 5c66d1b9b30f737fcef85a0b75bfe0590e16b62a ]

dequeue_task_rt() only decrements 'rt_rq->rt_nr_running' after having
called sched_update_tick_dependency() preventing it from re-enabling the
tick on systems that no longer have pending SCHED_RT tasks but have
multiple runnable SCHED_OTHER tasks:

  dequeue_task_rt()
    dequeue_rt_entity()
      dequeue_rt_stack()
        dequeue_top_rt_rq()
	  sub_nr_running()	// decrements rq->nr_running
	    sched_update_tick_dependency()
	      sched_can_stop_tick()	// checks rq->rt.rt_nr_running,
	      ...
        __dequeue_rt_entity()
          dec_rt_tasks()	// decrements rq->rt.rt_nr_running
	  ...

Every other scheduler class performs the operation in the opposite
order, and ...