claw: make use of DIV_ROUND_UP

The kernel.h macro DIV_ROUND_UP performs the computation
(((n) + (d) - 1) / (d)) but is perhaps more readable.

Signed-off-by: Ursula Braun <braunu@de.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>