unix: add dm-ioctl constants and types

This interface is used by devicemapper framework to manage its block
devices.

Change-Id: If86a679461867dac18b56bdf005567ffc4b7f951
Reviewed-on: https://go-review.googlesource.com/c/sys/+/247437
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/unix/ztypes_linux_s390x.go b/unix/ztypes_linux_s390x.go
index cca1b6b..349f483 100644
--- a/unix/ztypes_linux_s390x.go
+++ b/unix/ztypes_linux_s390x.go
@@ -117,6 +117,13 @@
 	_      [4]byte
 }
 
+type DmNameList struct {
+	Dev  uint64
+	Next uint32
+	Name [0]byte
+	_    [4]byte
+}
+
 const (
 	FADV_DONTNEED = 0x6
 	FADV_NOREUSE  = 0x7