And also do a better job of returning proper NET_{RX,XMIT}_ values.

Based on a patch and suggestions by Mark Smith.

This fixes CVE-2009-2903
Reported-by: NMark Smith <lk-netdev@lk-netdev.nosense.org>
Signed-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>