FROMLIST: cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1

If the scheduler cannot find an allowed CPU for a task,
cpuset_cpus_allowed_fallback() will widen the affinity to cpu_possible_mask
if cgroup v1 is in use.

In preparation for allowing architectures to provide their own fallback
mask, just return early if we're not using cgroup v2 and allow
select_fallback_rq() to figure out the mask by itself.

Cc: Li Zefan <lizefan@huawei.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Quentin Perret <qperret@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
[will: Check task_cpu_possible_mask() in v2 mode as per on-list comments]
Link: https://lore.kernel.org/r/20201217121552.ds7g2icvqp5nvtha@e107158-lin.cambridge.arm.com
Bug: 178507149
Link: https://lore.kernel.org/linux-arch/20201208132835.6151-8-will@kernel.org/


Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I7a6b234c28495bae6feeecceafb841aae4de6046