net: use peer when purging queue in qemu_flush_or_purge_queue_packets()

The sender of packet will be checked in the qemu_net_queue_purge() but
we use NetClientState not its peer when trying to purge the incoming
queue in qemu_flush_or_purge_packets(). This will trigger the assert
in virtio_net_reset since we can't pass the sender check:

hw/net/virtio-net.c:533: void virtio_net_reset(VirtIODevice *): Assertion
`!virtio_net_get_subqueue(nc)->async_tx.elem' failed.
CTSRD-CHERI#9 0x55a33fa31b78 in virtio_net_reset hw/net/virtio-net.c:533:13
CTSRD-CHERI#10 0x55a33fc88412 in virtio_reset hw/virtio/virtio.c:1919:9
CTSRD-CHERI#11 0x55a341d82764 in virtio_bus_reset hw/virtio/virtio-bus.c:95:9
CTSRD-CHERI#12 0x55a341dba2de in virtio_pci_reset hw/virtio/virtio-pci.c:1824:5
CTSRD-CHERI#13 0x55a341db3e02 in virtio_pci_common_write hw/virtio/virtio-pci.c:1252:13
CTSRD-CHERI#14 0x55a33f62117b in memory_region_write_accessor memory.c:496:5
CTSRD-CHERI#15 0x55a33f6205e4 in access_with_adjusted_size memory.c:557:18
CTSRD-CHERI#16 0x55a33f61e177 in memory_region_dispatch_write memory.c:1488:16

Reproducer:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg701914.html

Fix by using the peer.

Reported-by: "Alexander Bulekov" <alxndr@bu.edu>
Acked-by: Alexander Bulekov <alxndr@bu.edu>
Fixes: ca77d85 ("net: complete all queued packets on VM stop")
Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/net.c

@@ -610,7 +610,7 @@ void qemu_flush_or_purge_queued_packets(NetClientState *nc, bool purge)
         qemu_notify_event();
     } else if (purge) {
         /* Unable to empty the queue, purge remaining packets */
-        qemu_net_queue_purge(nc->incoming_queue, nc);
+        qemu_net_queue_purge(nc->incoming_queue, nc->peer);
     }
 }