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>

arch/s390/kernel/machine_kexec.c

@@ -54,7 +54,7 @@ static void __do_machine_kdump(void *image)
 	 * This need to be done *after* s390_reset_system set the
 	 * prefix register of this CPU to zero
 	 */
-	memcpy((void *) __LC_FPREGS_SAVE_AREA,
+	memcpy(absolute_pointer(__LC_FPREGS_SAVE_AREA),
 	       (void *)(prefix + __LC_FPREGS_SAVE_AREA), 512);
 
 	__load_psw_mask(PSW_MASK_BASE | PSW_DEFAULT_KEY | PSW_MASK_EA | PSW_MASK_BA);