sched_ext: fix application of sizeof to pointer

sizeof when applied to a pointer typed expression gives the size of
the pointer.

The proper fix in this particular case is to code sizeof(*cpuset)
instead of sizeof(cpuset).

This issue was detected with the help of Coccinelle.

Fixes: 22a92020 ("sched_ext: Implement tickless support")
Signed-off-by: guanjing <guanjing@cmss.chinamobile.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>