sched_ext: Convert ops.set_cmask() to arena-resident cmask

sched_ext: Convert ops.set_cmask() to arena-resident cmask

ops_cid.set_cmask() expects a cmask. The kernel couldn't write into the
arena, so it translated cpumask -> cmask in kernel memory and passed the
result as a trusted pointer. The BPF cmask helpers all operate on arena
cmasks though, so the BPF side had to word-by-word probe-read the kernel
cmask into an arena cmask via cmask_copy_from_kernel() before any helper
could touch it. It works, but is clumsy.

With direct kernel-side arena access now in place, build the cmask in the
arena. The kernel writes to it through the kern_va side of the dual mapping.
BPF directly dereferences it via an __arena pointer like any other arena
struct.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>