drm/nouveau/uvmm: fix NULL deref unwinding an OP_MAP_SPARSE op

Each bind_job_op is zeroed by kzalloc_obj() in bind_job_op_from_uop(),
and the OP_MAP_SPARSE case in nouveau_uvmm_bind_job_submit() only creates
a region, so op->ops stays NULL for a successfully processed sparse map.

If a later op in the same job fails, the reverse unwind loop revisits that
op and calls drm_gpuva_ops_free(&uvmm->base, op->ops) unconditionally.
drm_gpuva_ops_free() dereferences its argument right away
(list_for_each_entry_safe on &ops->list), so a NULL op->ops oopses. The
path is reachable by any render-node fd holder, since NOUVEAU_VM_BIND is
DRM_RENDER_ALLOW.

Guard the free with IS_ERR_OR_NULL(), as nouveau_uvmm_bind_job_cleanup()
already does for the identical free.

Fixes: b88baab828 ("drm/nouveau: implement new VM_BIND uAPI")
Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Assisted-by: Claude:claude-opus-5
Cc: stable@vger.kernel.org
Signed-off-by: Zhenhao Wan <whi4ed0g@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patch.msgid.link/20260811-nouveau-uvmm-vmbind-fixes-v2-1-aaee4b395d04@gmail.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

drivers/gpu/drm/nouveau/nouveau_uvmm.c

@@ -1489,7 +1489,8 @@ nouveau_uvmm_bind_job_submit(struct nouveau_job *job,
 			break;
 		}
 
-		drm_gpuva_ops_free(&uvmm->base, op->ops);
+		if (!IS_ERR_OR_NULL(op->ops))
+			drm_gpuva_ops_free(&uvmm->base, op->ops);
 		op->ops = NULL;
 		op->reg = NULL;
 	}