Create a lightweight callback interface to allow idxd sub-drivers to
be notified when work sent to idxd wqs has completed.

For a sub-driver to be notified of work completion, it needs to:

  - Set the descriptor's 'Request Completion Interrupt'
    (IDXD_OP_FLAG_RCI)

  - Set the sub-driver desc_complete() callback when registering the
    sub-driver e.g.:

      struct idxd_device_driver my_drv = {
            .probe = my_probe,
            .desc_complete = my_complete,
      }

  - Set the sub-driver-specific context in the sub-driver's descriptor
    e.g:

      idxd_desc->crypto.req = req;
      idxd_desc->crypto.tfm = tfm;
      idxd_desc->crypto.src_addr = src_addr;
      idxd_desc->crypto.dst_addr = dst_addr;

When the work completes and the completion irq fires, idxd will invoke
the desc_complete() callback with pointers to the descriptor, context,
and completion_type.
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Reviewed-by: Fenghua Yu <fenghua.yu@intel.com>
Acked-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>