sungem: 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
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/sungem.c

@@ -306,7 +306,7 @@ static void sungem_send_packet(SunGEMState *s, const uint8_t *buf,
     NetClientState *nc = qemu_get_queue(s->nic);
 
     if (s->macregs[MAC_XIFCFG >> 2] & MAC_XIFCFG_LBCK) {
-        nc->info->receive(nc, buf, size);
+        qemu_receive_packet(nc, buf, size);
     } else {
         qemu_send_packet(nc, buf, size);
     }