VOP is broken in mainline since commit 1ce9e605 ("virtio_ring:
introduce packed ring support"); attempting to use the virtqueues leads
to various kernel crashes.  I'm testing it with my not-yet-merged
loopback patches, but even the in-tree MIC hardware cannot work.

The problem is not in the referenced commit per se, but is due to the
following hack in vop_find_vq() which depends on the layout of private
structures in other source files, which that commit happened to change:

  /*
   * To reassign the used ring here we are directly accessing
   * struct vring_virtqueue which is a private data structure
   * in virtio_ring.c. At the minimum, a BUILD_BUG_ON() in
   * vring_new_virtqueue() would ensure that
   *  (&vq->vring == (struct vring *) (&vq->vq + 1));
   */
  vr = (struct vring *)(vq + 1);
  vr->used = used;

Fix vop by using __vring_new_virtqueue() to create the needed vring
layout from the start, instead of attempting to patch in the used ring
later.  __vring_new_virtqueue() was added way back in commit
2a2d1382 ("virtio: Add improved queue allocation API") in order to
address mic's usecase, according to the commit message.

Fixes: 1ce9e605 ("virtio_ring: introduce packed ring support")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>