sched_ext: Remove redundant css_put() in scx_cgroup_init()

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

sched_ext: Remove redundant css_put() in scx_cgroup_init()

The iterator css_for_each_descendant_pre() walks the cgroup hierarchy

under cgroup_lock(). It does not increment the reference counts on

yielded css structs.

According to the cgroup documentation, css_put() should only be used

to release a reference obtained via css_get() or css_tryget_online().

Since the iterator does not use either of these to acquire a reference,

calling css_put() in the error path of scx_cgroup_init() causes a

refcount underflow.

Remove the unbalanced css_put() to prevent a potential Use-After-Free

(UAF) vulnerability.