sched/core: Prevent (user) space tasks from affining to isolated cpus

We don't want user space tasks to run on isolated cpus. If the affinity
mask that the user space task is trying to set only includes online
cpus that are isolated return error.

Also ensure that tasks do not get stuck on isolated cores. We are not
properly updating the mask that we check against the current CPU so we
might end up thinking we can run on the current CPU. Fix this.

Change-Id: I078d01e63860d1fc60fc96eb0c739c0f680ae983
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>