Revert "sched/deadline: Remove cpu_active_mask from cpudl_find()"

This reverts commit 9659e1ee. The change leaves a possibility of
scheduling dl task on offline cpu. The original commit text suggests
that it should not be a problem since migrate_tasks() will take care of
it, but there can be delay between marking cpu as offline and
migrate_tasks() call since they are called on different hotplug
notifications-offline happens on CPUHP_TEARDOWN_CPU while
migrate_tasks() gets called on CPUHP_AP_SCHED_STARTING. And in
between this duration there is a chance the dl task can get scheduled on
this offline cpu and lead to BUG_ON. Hence revert the change.

Change-Id: I6f4ea10d2ce4e83e12d36b9b90b1b630ffe4fc74
[pujag@codeaurora.org: fix minor conflicts due to reorg of the code]
Signed-off-by: Puja Gupta <pujag@codeaurora.org>