s390/kexec: Disable stack protector in s390_reset_system()

s390/kexec: Disable stack protector in s390_reset_system()

s390_reset_system() calls set_prefix(0), which switches back to the
absolute lowcore. At that point the stack protector canary no longer
matches the canary from the lowcore the function was entered with, so
the stack check fails.

Mark s390_reset_system() __no_stack_protector. This is safe here since
its callers (__do_machine_kdump() and __do_machine_kexec()) are
effectively no-return and fall back to disabled_wait() on failure.

Fixes: f5730d44e05e ("s390: Add stackprotector support")
Reported-by: Nikita Dubrovskii <nikita@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>