core/cpu: Call memset with proper cpu_thread offset
core/cpu: Call memset with proper cpu_thread offset

"cpu_thread *t + value" vs "(void *)t + val"

Fixes: cfe9d441 (core/cpu: Prevent clobbering of stack guard for boot-cpu)
CC: stable <015da030a3546f35003970f5a8d22f752be7c14a@lists.ozlabs.org> # v6.0+
CC: Vaibhav Jain <e78384dfaa97bf0f3945c4da4075af8ac33ade0a@linux.vnet.ibm.com>
CC: Nicholas Piggin <24829078df5003d9dd1dd871ca2aeb0b1cf95ef4@gmail.com>
CC: Mahesh Salgaonkar <6a890e1065a1d4e8df9dcf1d9a2c9c740bbcf8aa@linux.vnet.ibm.com>
Acked-by: Mahesh Salgaonkar <6a890e1065a1d4e8df9dcf1d9a2c9c740bbcf8aa@linux.vnet.ibm.com>
Signed-off-by: Vasant Hegde <a6d753843573f7bfed5075c1e875b0be8007a0ab@linux.vnet.ibm.com>
Reviewed-by: Vaibhav Jain<e78384dfaa97bf0f3945c4da4075af8ac33ade0a@linux.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.ibm.com>

open-power/skiboot,legoater/skiboot,stewart-ibm/skiboot,qemu/skiboot,open-power/skiboot,stewart-ibm/skiboot,legoater/skiboot,open-power/skiboot,shenki/skiboot,legoater/skiboot,open-power/skiboot,shenki/skiboot,qemu/skiboot,stewart-ibm/skiboot,qemu/skiboot,shenki/skiboot,open-power/skiboot,shenki/skiboot,qemu/skiboot,legoater/skiboot,shenki/skiboot,legoater/skiboot,qemu/skiboot