drm/msm: RCU-free the scheduler-containing ring and VM objects

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

drm/msm: RCU-free the scheduler-containing ring and VM objects

Both struct msm_ringbuffer and struct msm_gem_vm embed a struct
drm_gpu_scheduler. msm_ringbuffer_destroy() and the VM free callback
msm_gem_vm_free() call drm_sched_fini() on the embedded scheduler and then
free the containing object with plain kfree().

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 (the submit out-fence, or
a VM_BIND fence, via sync_file / drm_syncobj) keeps pointing at the embedded
scheduler after the ring/VM is freed, so a later get_timeline_name() --
reachable unprivileged through SYNC_IOC_FILE_INFO -- dereferences freed slab
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
objects with kfree_rcu() instead of kfree().

Patchwork: https://patchwork.freedesktop.org/patch/750234/