net: rtl8139: limit processing of ring descriptors

RTL8139 ethernet controller in C+ mode supports multiple
descriptor rings, each with maximum of 64 descriptors. While
processing transmit descriptor ring in 'rtl8139_cplus_transmit',
it does not limit the descriptor count and runs forever. Add
check to avoid it.

Reported-by: Andrew Henderson <hendersa@icculus.org>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/rtl8139.c

@@ -2350,7 +2350,7 @@ static void rtl8139_cplus_transmit(RTL8139State *s)
 {
     int txcount = 0;
 
-    while (rtl8139_cplus_transmit_one(s))
+    while (txcount < 64 && rtl8139_cplus_transmit_one(s))
     {
         ++txcount;
     }