net/virtio: add barrier before reading the flags

net/virtio: add barrier before reading the flags

Reading the used->flags could be reordered with avail->idx update.
vhost in kernel disables notifications for the time of packets
receiving, like this:

    1. disable notify
    2. process packets
    3. enable notify
    4. has more packets ? goto 1

In case of reordering, virtio driver could read the flags on
step 2 while notifications disabled and update avail->idx after
the step 4, i.e. vhost will exit the loop on step 4 with
notifications enabled, but virtio will not notify.

Fixes: c1f86306a026 ("virtio: add new driver")
Cc: stable@dpdk.org
Reported-by: Shahaf Shuler <shahafs@mellanox.com>
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>