tg3: Prevent rx producer ring overruns

When operating in RSS mode, it is possible for one rx return ring to
submit enough rx buffers back to the hardware such that it inadvertently
overwrites data needed by another rx return ring.  This patch addresses
the problem by looking for non-NULL skb pointers in the
rx_[std|jmb]_buffers rings that parallel the rx producer rings.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>