sched: fix warning in sched_setaffinity

Commit 8f9ea86fdf99b added some logic to sched_setaffinity that included
a WARN when a per-task affinity assignment races with a cpuset update.

Specifically, we can have a race where a cpuset update results in the
task affinity no longer being a subset of the cpuset. That's fine; we
have a fallback to instead use the cpuset mask. However, we have a WARN
set up that will trigger if the cpuset mask has no overlap at all with
the requested task affinity. This shouldn't be a warning condition; its
trivial to create this condition.

Reproduced the warning by the following setup:

- $PID inside a cpuset cgroup
- another thread repeatedly switching the cpuset cpus from 1-2 to just 1
- another thread repeatedly setting the $PID affinity (via taskset) to 2

Fixes: 8f9ea86fdf99b ("sched: Always preserve the user requested cpumask")
Signed-off-by: Josh Don <[email protected]>
Acked-and-tested-by: Vincent Guittot <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Waiman Long <[email protected]>
Tested-by: Madadi Vineeth Reddy <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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