sched/cpupri: Exclude isolated CPUs from the lowest_mask

The cpupri_find() returns the candidate CPUs which are running
lower priority than the waking RT task in the lowest_mask. This
contains isolated CPUs as well. Since the energy aware CPU selection
skips isolated CPUs, no target CPU may be found if all unisolated CPUs
are running higher priority RT tasks. In which case, we fallback to
the default CPU selection algorithm and returns an isolated CPU. This
decision is reversed by select_task_rq() and returns an unisolated
CPU that is busy with other RT tasks. This RT task packing is desired
behavior. However, RT push mechanism pushes the packed RT task to
an isolated CPU. This can be avoided by excluding isolated CPUs from
the lowest_mask returned by cpupri_find().

Change-Id: I75486b3935caf496a638d0333565beffc47fe249
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>

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