net/virtio: add barriers for extra descriptors on Rx split

net/virtio: add barriers for extra descriptors on Rx split

There should be read barrier between checking VIRTQUEUE_NUSED (reading
the used->idx) and reading these descriptors. It's done for the first
checks at the beginning of these functions but missed while checking
for extra required descriptors.

Fixes: e5f456a98d3c ("net/virtio: support in-order Rx and Tx")
Fixes: 13ce5e7eb94f ("virtio: mergeable buffers")
Cc: stable@dpdk.org
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>