pcnet: switch to use qemu_receive_packet() for loopback

This patch switches to use qemu_receive_packet() which can detect
reentrancy and return early.

This is intended to address CVE-2021-3416.

Cc: Prasad J Pandit <ppandit@redhat.com>
Cc: qemu-stable@nongnu.org
Buglink: https://bugs.launchpad.net/qemu/+bug/1917085
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com
Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/pcnet.c

@@ -1250,7 +1250,7 @@ txagain:
             if (BCR_SWSTYLE(s) == 1)
                 add_crc = !GET_FIELD(tmd.status, TMDS, NOFCS);
             s->looptest = add_crc ? PCNET_LOOPTEST_CRC : PCNET_LOOPTEST_NOCRC;
-            pcnet_receive(qemu_get_queue(s->nic), s->buffer, s->xmit_pos);
+            qemu_receive_packet(qemu_get_queue(s->nic), s->buffer, s->xmit_pos);
             s->looptest = 0;
         } else {
             if (s->nic) {