sched/core: Fix rq lock pinning warning after call balance callbacks

sched/core: Fix rq lock pinning warning after call balance callbacks

This can be reproduced by running rt-migrate-test:

 WARNING: CPU: 2 PID: 2195 at kernel/locking/lockdep.c:3670 lock_unpin_lock()
 unpinning an unpinned lock
 ...
 Call Trace:
  dump_stack()
  __warn()
  warn_slowpath_fmt()
  lock_unpin_lock()
  __balance_callback()
  __schedule()
  schedule()
  futex_wait_queue_me()
  futex_wait()
  do_futex()
  SyS_futex()
  do_syscall_64()
  entry_SYSCALL64_slow_path()

Revert the rq_lock_irqsave() usage here, the whole point of the
balance_callback() was to allow dropping rq->lock.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 8a8c69c32778 ("sched/core: Add rq->lock wrappers")
Link: http://lkml.kernel.org/r/1489718719-3951-1-git-send-email-wanpeng.li@hotmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>