s390/checksum: Fix csum_partial() without vector facility

s390/checksum: Fix csum_partial() without vector facility

Currently csum_partial() calls csum_copy() with copy=false and dst=NULL.
On machines without the vector facility, csum_copy() falls back to
cksm(dst, ...), causing the checksum to be calculated from address zero
instead of the source buffer.

The VX implementation already checksums data loaded from src. Make the
fallback do the same by passing src to cksm().

Fixes: dcd3e1de9d17 ("s390/checksum: provide csum_partial_copy_nocheck()")
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>