[PATCH v4 1/2] sched/deadline: Add cpudl_maximum_dl() for better readability

[Byungchul Park]

Changes from v3
 - Rebase onto the latest tip/sched/core
 - Apply what Joel suggests, to set cp->elements[0].cpu to -1
   when the heap becomes empty in cpudl_clear()

Changes from v2
 - Run spellchecker over the text and fix typos
 - Add acked-by Daniel

Changes from v1
 - Enhance commit msg
 - Prevent WARN in cpumask_test_cpu() in cpudl_find() when best_cpu == -1

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