sched_ext: Read scx_root under scx_cgroup_ops_rwsem in cgroup setters

In the Linux kernel, the following vulnerability has been resolved:

sched_ext: Read scx_root under scx_cgroup_ops_rwsem in cgroup setters

scx_group_set_{weight,idle,bandwidth}() cache scx_root before acquiring

scx_cgroup_ops_rwsem, so the pointer can be stale by the time the op runs.

If the loaded scheduler is disabled and freed (via RCU work) and another is

enabled between the naked load and the rwsem acquire, the reader sees

scx_cgroup_enabled=true (the new scheduler's) but dereferences the freed one

- UAF on SCX_HAS_OP(sch, ...) / SCX_CALL_OP(sch, ...).

scx_cgroup_enabled is toggled only under scx_cgroup_ops_rwsem write

(scx_cgroup_{init,exit}), so reading scx_root inside the rwsem read section

correlates @sch with the enabled snapshot.