HTB qdisc overlimits counter is properly increased, but we have no per
class counter, meaning it is difficult to diagnose HTB problems.

This patch adds this counter, visible in "tc -s class show dev eth0",
with current iproute2.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: NDenys Fedoryshchenko <nuclearcat@nuclearcat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>