KVM: fix build warning within __kvm_set_memory_region() on s390

Get rid of this warning:

  CC      arch/s390/kvm/../../../virt/kvm/kvm_main.o
arch/s390/kvm/../../../virt/kvm/kvm_main.c:596:12: warning: 'kvm_create_dirty_bitmap' defined but not used

The only caller of the function is within a !CONFIG_S390 section, so add the
same ifdef around kvm_create_dirty_bitmap() as well.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

virt/kvm/kvm_main.c

@@ -588,6 +588,7 @@ static int kvm_vm_release(struct inode *inode, struct file *filp)
 	return 0;
 }
 
+#ifndef CONFIG_S390
 /*
  * Allocation size is twice as large as the actual dirty bitmap size.
  * This makes it possible to do double buffering: see x86's
@@ -608,6 +609,7 @@ static int kvm_create_dirty_bitmap(struct kvm_memory_slot *memslot)
 	memslot->dirty_bitmap_head = memslot->dirty_bitmap;
 	return 0;
 }
+#endif /* !CONFIG_S390 */
 
 /*
  * Allocate some memory and give it an address in the guest physical address