sched: Make resched_cpu() unconditional

The current implementation of synchronize_sched_expedited() incorrectly
assumes that resched_cpu() is unconditional, which it is not.  This means
that synchronize_sched_expedited() can hang when resched_cpu()'s trylock
fails as follows (analysis by Neeraj Upadhyay):

o    CPU1 is waiting for expedited wait to complete:
sync_rcu_exp_select_cpus
     rdp->exp_dynticks_snap & 0x1   // returns 1 for CPU5
     IPI sent to CPU5

synchronize_sched_expedited_wait
         ret = swait_event_timeout(
                                     rsp->expedited_wq,
  sync_rcu_preempt_exp_done(rnp_root),
                                     jiffies_stall);

            expmask = 0x20 , and CPU 5 is in idle path (in cpuidle_enter())

o    CPU5 handles IPI and fails to acquire rq lock.

Handles IPI
     sync_sched_exp_handler
         resched_cpu
             returns while failing to try lock acquire rq->lock
         need_resched is not set

o    CPU5 calls  rcu_idle_enter() and as need_resched is not set, goes to
     idle (schedule() is not called).

o    CPU 1 reports RCU stall.

Given that resched_cpu() is now used only by RCU, this commit fixes the
assumption by making resched_cpu() unconditional.

Change-Id: I67cbf28612004f4b78e355dd00b5abdd0f31ec13
Reported-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Suggested-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Patch-mainline: linux-kernel @ 18/09/17, 09:01
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>