s390x: user ctl_set/clear_bit for low address protection

User the provided helper functions now.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180824115059.1517-2-david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>