virtio: remove unnecessary thread fence while reading next descriptor

It was supposed to be a compiler barrier and it was a compiler barrier
initially called 'wmb' when virtio core support was introduced.
Later all the instances of 'wmb' were switched to smp_wmb to fix memory
ordering issues on non-x86 platforms.  However, this one doesn't need
to be an actual barrier, as its only purpose was to ensure that the
value is not read twice.

And since commit aa570d6 ("virtio: combine the read of a descriptor")
there is no need for a barrier at all, since we're no longer reading
guest memory here, but accessing a local structure.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Message-Id: <20230927140016.2317404-2-i.maximets@ovn.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio.c

@@ -1060,8 +1060,6 @@ static int virtqueue_split_read_next_desc(VirtIODevice *vdev, VRingDesc *desc,
 
     /* Check they're not leading us off end of descriptors. */
     *next = desc->next;
-    /* Make sure compiler knows to grab that: we don't want it changing! */
-    smp_wmb();
 
     if (*next >= max) {
         virtio_error(vdev, "Desc next is %u", *next);