CVE-2026-53328: sched_ext: Don't warn on NULL cgrp_moving_from in scx_cgroup_move_task()

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

schedext: Don't warn on NULL cgrpmovingfrom in scxcgroupmovetask()

A WARN fires when systemd's user manager writes "+cpu +memory +pids" to its own subtreecontrol while a schedext scheduler is loaded:

WARNING: at kernel/sched/ext.c:3227 scxcgroupmovetask+0xa8/0xb0 scxcgroupmovetask+0xa8/0xb0 schedmovetask+0x134/0x290 cpucgroupattach+0x39/0x70 cgroupmigrateexecute+0x37d/0x450 cgroupupdatedflcsses+0x1e3/0x270 cgroupsubtreecontrolwrite+0x3e7/0x440

scxcgroupcanattach() arms cgrpmovingfrom only when a task's cpu cgroup changes. It can still be NULL when scxcgroupmovetask() runs, through this sequence:

Step Result --------------------------------- ---------------------------------- 1. cpu enabled on cgroup G cpu css = A 2. cpu toggled off then on for G A killed, B created (same cgroup) 3. an exiting task keeps A alive migration skips it, A now stale 4. +memory migrates G stale A vs current B pulls cpu in 5. cpu attach runs for all tasks hits a live, cpu-unchanged task 6. scxcgroupmovetask() on it cgrpmovingfrom NULL -> WARN

The mismatch is that scxcgroupcanattach() keys on cgroup identity while migration drives the move on css identity, so a NULL cgrpmovingfrom here is a legitimate css-only migration, not a missing prep.

The call is already gated on cgrpmovingfrom, so just drop the warning. ops.cgroupprepmove() and ops.cgroupmove() stay paired.