[S390] Use csum_partial in checksum.h

The cksm function in system.h is duplicate to csum_partial in checksum.h.
Remove cksm and use csum_partial instead.

Signed-off-by: Frank Munzert <munzert@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>