s390/cpum_sf: Use DIV_ROUND_UP

Use macro DIV_ROUND_UP() for calculation of number of SDBT
SDBT pages required for index pages. This macro is already
used throughout the file.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>