net/virtio: fix memory leak when reinitializing device

net/virtio: fix memory leak when reinitializing device

Free the previous queues and the attached mbufs before initializing new
ones.

The function virtio_dev_free_mbufs() is now called when reconfiguring the
device, so we also need to add a check to ensure that it won't crash for
uninitialized queues.

Cc: stable@dpdk.org
Fixes: 60e6f4707ef2 ("net/virtio: reinitialize device when configuring")

Signed-off-by: Zijie Pan <zijie.pan@6wind.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Yuanhan Liu <yliu@fridaylinux.org>