sched: core: Exclude isolated cpus in sched_getaffinity cpumask

User space tasks will have the same semantics for hotplug and isolation.
Currently the sched_getaffinity() include isolated CPU on which user
space tasks are forbidden to run. So fix this by excluding isolated cpus
while returning cpu mask for user space tasks in sched_getaffinity().

Change-Id: Ib6a8ca864754f73bbaf809c3ceaef5f91d567ca9
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>