[PKT_SCHED] HTB: initialize upper bound properly

The upper bound for HTB time diff needs to be scaled to PSCHED
units rather than just assuming usecs.  The field mbuffer is used
in TDIFF_SAFE(), as an upper bound.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Acked-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>