sched/dl: Simplify pick_dl_task()

1) Nobody calls pick_dl_task() with negative cpu, it's old RT leftover.

2) If p->nr_cpus_allowed is 1, than the affinity has just been changed
  in set_cpus_allowed_ptr(); we'll pick it just earlier than migration
  thread.

Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1410529340.3569.27.camel@tkhai


Signed-off-by: Ingo Molnar <mingo@kernel.org>