[NET_SCHED]: Make HTB scheduler work with TSO.

Currently the HTB scheduler does not correctly account for TSO packets
which causes large inaccuracies in the bandwidth control when using TSO.
This patch allows the HTB scheduler to work with TSO enabled devices.

Signed-off-by: Ranjit Manomohan <ranjitm@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>