virtio_mmio: fix off by one error allocating queue

vm_setup_vq fails to allow VirtQueues needing only 2 pages of
storage, as it should. Found with a kernel using 64kB pages, but
can be provoked if a virtio device reports QueueNumMax where the
descriptor table and available ring fit in one page, and the used
ring on the second (<= 227 descriptors with 4kB pages and <= 3640
with 64kB pages.)

Signed-off-by: Brian Foley <brian.foley@arm.com>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/virtio_mmio.c

@@ -334,8 +334,8 @@ static struct virtqueue *vm_setup_vq(struct virtio_device *vdev, unsigned index,
 	while (1) {
 		size = PAGE_ALIGN(vring_size(info->num,
 				VIRTIO_MMIO_VRING_ALIGN));
-		/* Already smallest possible allocation? */
-		if (size <= VIRTIO_MMIO_VRING_ALIGN * 2) {
+		/* Did the last iter shrink the queue below minimum size? */
+		if (size < VIRTIO_MMIO_VRING_ALIGN * 2) {
 			err = -ENOMEM;
 			goto error_alloc_pages;
 		}