s390/kexec: silence -Warray-bounds warning

Just use absolute_pointer() like e.g. in commit 545c2722 ("alpha:
Silence -Warray-bounds warnings") to get rid of this warning:

arch/s390/kernel/machine_kexec.c:59:9: warning: ‘memcpy’ offset [0, 511] is out of the bounds [0, 0] [-Warray-bounds]

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>