drm/amdgpu: don't access invalid sched

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

drm/amdgpu: don't access invalid sched

Since 2320c9e6a768 ("drm/sched: memset() 'job' in drm_sched_job_init()")

accessing job->base.sched can produce unexpected results as the initialisation

of (*job)->base.sched done in amdgpu_job_alloc is overwritten by the

memset.

This commit fixes an issue when a CS would fail validation and would

be rejected after job->num_ibs is incremented. In this case,

amdgpu_ib_free(ring->adev, ...) will be called, which would crash the

machine because the ring value is bogus.

To fix this, pass a NULL pointer to amdgpu_ib_free(): we can do this

because the device is actually not used in this function.

The next commit will remove the ring argument completely.

(cherry picked from commit 2ae520cb12831d264ceb97c61f72c59d33c0dbd7)