msf2-mac: 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: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/msf2-emac.c

@@ -158,7 +158,7 @@ static void msf2_dma_tx(MSF2EmacState *s)
          * R_CFG1 bit 0 is set.
          */
         if (s->regs[R_CFG1] & R_CFG1_LB_EN_MASK) {
-            nc->info->receive(nc, buf, size);
+            qemu_receive_packet(nc, buf, size);
         } else {
             qemu_send_packet(nc, buf, size);
         }