sched: Provide rt_mutex specific scheduler helpers

With PREEMPT_RT there is a rt_mutex recursion problem where
sched_submit_work() can use an rtlock (aka spinlock_t). More
specifically what happens is:

  mutex_lock() /* really rt_mutex */
    ...
      __rt_mutex_slowlock_locked()
	task_blocks_on_rt_mutex()
          // enqueue current task as waiter
          // do PI chain walk
        rt_mutex_slowlock_block()
          schedule()
            sched_submit_work()
              ...
              spin_lock() /* really rtlock */
                ...
                  __rt_mutex_slowlock_locked()
                    task_blocks_on_rt_mutex()
                      // enqueue current task as waiter *AGAIN*
                      // *CONFUSION*

Fix this by making rt_mutex do the sched_submit_work() early, before
it enqueues itself as a waiter -- before it even knows *if* it will
wait.

[[ basically Thomas' patch but with different naming and a few asserts
   added ]]

Originally-by: Thomas Gleixner <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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