unix: add devlink Linux generic netlink API constants

For more information on devlink, see: https://lwn.net/Articles/674867/.

Change-Id: I68815c94160864f4a3d2bb78b2e5d06949d27acf
Reviewed-on: https://go-review.googlesource.com/c/sys/+/203878
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/mkerrors.sh

@@ -190,6 +190,7 @@ struct ltchars {
 #include <linux/can.h>
 #include <linux/capability.h>
 #include <linux/cryptouser.h>
+#include <linux/devlink.h>
 #include <linux/errqueue.h>
 #include <linux/falloc.h>
 #include <linux/fanotify.h>
@@ -524,6 +525,7 @@ ccflags="$@"
 		$2 ~ /^(HDIO|WIN|SMART)_/ ||
 		$2 ~ /^CRYPTO_/ ||
 		$2 ~ /^TIPC_/ ||
+		$2 ~ /^DEVLINK_/ ||
 		$2 !~ "WMESGLEN" &&
 		$2 ~ /^W[A-Z0-9]+$/ ||
 		$2 ~/^PPPIOC/ ||