net/cadence_gem: Fix rx multi-fragment packets

Bytes_to_copy was being updated before its final use where it
advances the rx buffer pointer. This was causing total mayhem,
where packet data for any subsequent fragments was being fetched
from the wrong place.
Reported-by: NDeepika Dhamija <deepika@xilinx.com>
Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Message-id: c2a1c65c1fd06eb274442a0fa4a6839d940e145e.1386136219.git.peter.crosthwaite@xilinx.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/net/cadence_gem.c

@@ -733,8 +733,8 @@ static ssize_t gem_receive(NetClientState *nc, const uint8_t *buf, size_t size)
         /* Copy packet data to emulated DMA buffer */
         cpu_physical_memory_write(rx_desc_get_buffer(s->rx_desc) + rxbuf_offset,
                                   rxbuf_ptr, MIN(bytes_to_copy, rxbufsize));
-        bytes_to_copy -= MIN(bytes_to_copy, rxbufsize);
         rxbuf_ptr += MIN(bytes_to_copy, rxbufsize);
+        bytes_to_copy -= MIN(bytes_to_copy, rxbufsize);
 
         /* Update the descriptor.  */
         if (first_desc) {