stop_machine, sched: Fix migrate_swap() vs. active_balance() deadlock

Matt reported the following deadlock:

CPU0					CPU1

schedule(.prev=migrate/0)		<fault>
  pick_next_task()			  ...
    idle_balance()			    migrate_swap()
      active_balance()			      stop_two_cpus()
						spin_lock(stopper0->lock)
						spin_lock(stopper1->lock)
						ttwu(migrate/0)
						  smp_cond_load_acquire() -- waits for schedule()
        stop_one_cpu(1)
	  spin_lock(stopper1->lock) -- waits for stopper lock

Fix this deadlock by taking the wakeups out from under stopper->lock.
This allows the active_balance() to queue the stop work and finish the
context switch, which in turn allows the wakeup from migrate_swap() to
observe the context and complete the wakeup.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reported-by: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180420095005.GH4064@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>