s390/kprobes: use is_kernel() helper

Use is_kernel() helper instead of is_kernel_addr().

[hca@linux.ibm.com: add missing unsigned long cast]
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>