cadence_gem: 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: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/cadence_gem.c

@@ -1275,8 +1275,8 @@ static void gem_transmit(CadenceGEMState *s)
                 /* Send the packet somewhere */
                 if (s->phy_loop || (s->regs[GEM_NWCTRL] &
                                     GEM_NWCTRL_LOCALLOOP)) {
-                    gem_receive(qemu_get_queue(s->nic), s->tx_packet,
-                                total_bytes);
+                    qemu_receive_packet(qemu_get_queue(s->nic), s->tx_packet,
+                                        total_bytes);
                 } else {
                     qemu_send_packet(qemu_get_queue(s->nic), s->tx_packet,
                                      total_bytes);