sched: core: Fix possible hotplug race in set_cpus_allowed_ptr

Since a CPU may go offline after cpu_active_mask is used
to query active CPUs, set_cpus_allowed_ptr might inadverntently
pass an invalid cpu number to move_queued_task.

Fix this by ensuring that the cpumask op that uses cpu_active_mask
checks the return value.

Change-Id: Id43a629b40b72cc47773e4027d30953b3a94058d
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>