drm/nouveau: RCU-free the scheduler-containing nouveau_sched

struct nouveau_sched embeds a struct drm_gpu_scheduler (base).
nouveau_sched_destroy() calls nouveau_sched_fini() (which does
drm_sched_fini(&sched->base)) and then frees the object with plain
kfree(sched).

drm_sched_fence_get_timeline_name() returns fence->sched->name, and the
scheduler fence keeps a .release callback so it is not ops-detached on
signalling.  A finished fence exported to userspace via drm_syncobj /
sync_file therefore keeps pointing at &sched->base after nouveau_sched_destroy(),
and a later get_timeline_name() -- reachable unprivileged through
SYNC_IOC_FILE_INFO -- dereferences freed memory (KASAN slab-use-after-free
read).

Per the dma-fence lifetime contract the exporter must keep the data backing a
signalled fence alive for an RCU grace period.  Free the scheduler-containing
object with kfree_rcu() instead of kfree().

Fixes: 5f03a507b29e ("drm/nouveau: implement 1:1 scheduler - entity relationship")
Cc: stable@vger.kernel.org
Signed-off-by: Jonghyuk Kim(MalHyuk) <malhyuk97@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patch.msgid.link/20260902012717.880724-1-malhyuk97@gmail.com

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