unix: add Linux network namespace constants

This adds the NETNSA constants from net_namespace.h, used for creating
and querying network namespace IDs.

Fixes golang/go#26330
Change-Id: I786c4c4771be3eddc4fdb830a52e128aa1a2b9fd
Reviewed-on: https://go-review.googlesource.com/124436
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/linux/types.go

@@ -74,6 +74,7 @@ package unix
 #include <linux/hdreg.h>
 #include <linux/rtc.h>
 #include <linux/blkpg.h>
+#include <linux/net_namespace.h>
 
 // abi/abi.h generated by mkall.go.
 #include "abi/abi.h"
@@ -1539,3 +1540,13 @@ const (
 	BLKPG_DEL_PARTITION    = C.BLKPG_DEL_PARTITION
 	BLKPG_RESIZE_PARTITION = C.BLKPG_RESIZE_PARTITION
 )
+
+// netlink namespace
+// generated from
+// perl -nlE '/^\s*(NETNSA\w+)/ && say "$1 = C.$1"' /usr/include/linux/net_namespace.h
+const (
+	NETNSA_NONE = C.NETNSA_NONE
+	NETNSA_NSID = C.NETNSA_NSID
+	NETNSA_PID  = C.NETNSA_PID
+	NETNSA_FD   = C.NETNSA_FD
+)