sched/deadline: Unthrottle PI boosted threads while enqueuing

stress-ng has a test (stress-ng --cyclic) that creates a set of threads
under SCHED_DEADLINE with the following parameters:

    dl_runtime   =  10000 (10 us)
    dl_deadline  = 100000 (100 us)
    dl_period    = 100000 (100 us)

These parameters are very aggressive. When using a system without HRTICK
set, these threads can easily execute longer than the dl_runtime because
the throttling happens with 1/HZ resolution.

During the main part of the test, the system works just fine because
the workload does not try to run over the 10 us. The problem happens at
the end of the test, on the exit() path. During exit(), the threads need
to do some cleanups that require real-time mutex locks, mainly those
related to memory management, resulting in this scenario:

Note: locks are rt_mutexes...
 ------------------------------------------------------------------------
    TASK A:		TASK B:				TASK C:
    activation
							activation
			activation

    lock(a): OK!	lock(b): OK!
    			<overrun runtime>
    			lock(a)
    			-> block (task A owns it)
			  -> self notice/set throttled
 +--<			  -> arm replenished timer
 |    			switch-out
 |    							lock(b)
 |    							-> <C prio > B prio>
 |    							-> boost TASK B
 |  unlock(a)						switch-out
 |  -> handle lock a to B
 |    -> wakeup(B)
 |      -> B is throttled:
 |        -> do not enqueue
 |     switch-out
 |
 |
 +---------------------> replenishment timer
			-> TASK B is boosted:
			  -> do not enqueue
 ------------------------------------------------------------------------

BOOM: TASK B is runnable but !enqueued, holding TASK C: the system
crashes with hung task C.

This problem is avoided by removing the throttle state from the boosted
thread while boosting it (by TASK A in the example above), allowing it to
be queued and run boosted.

The next replenishment will take care of the runtime overrun, pushing
the deadline further away. See the "while (dl_se->runtime <= 0)" on
replenish_dl_entity() for more information.

Reported-by: Mark Simmons <msimmons@redhat.com>
Signed-off-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Juri Lelli <juri.lelli@redhat.com>
Tested-by: Mark Simmons <msimmons@redhat.com>
Link: https://lkml.kernel.org/r/5076e003450835ec74e6fa5917d02c4fa41687e6.1600170294.git.bristot@redhat.com