net/virtio: add barrier in interrupt enable

net/virtio: add barrier in interrupt enable

Typically, after enabling Rx interrupt, a check should be done
to make sure that there is no new incoming packets before going
to sleep. So a barrier is needed to make sure that any following
check won't happen before the interrupt is actually enabled.

Fixes: c056be239db5 ("net/virtio: add Rx interrupt enable/disable functions")
Cc: stable@dpdk.org
Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>