unix: add ethtool generic netlink constants for Linux

For more information about this interface, see:
https://www.kernel.org/doc/html/latest/networking/ethtool-netlink.html.

Change-Id: I4fe37e7dce3c52d72286bee047de2b49fe559b13
Reviewed-on: https://go-review.googlesource.com/c/sys/+/282812
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Matt Layher <mdlayher@gmail.com>

unix/mkerrors.sh

@@ -204,6 +204,7 @@ struct ltchars {
 #include <linux/devlink.h>
 #include <linux/dm-ioctl.h>
 #include <linux/errqueue.h>
+#include <linux/ethtool_netlink.h>
 #include <linux/falloc.h>
 #include <linux/fanotify.h>
 #include <linux/filter.h>
@@ -563,6 +564,7 @@ ccflags="$@"
 		$2 ~ /^TIPC_/ ||
 		$2 !~  "DEVLINK_RELOAD_LIMITS_VALID_MASK" &&
 		$2 ~ /^DEVLINK_/ ||
+		$2 ~ /^ETHTOOL_/ ||
 		$2 ~ /^LWTUNNEL_IP/ ||
 		$2 !~ "WMESGLEN" &&
 		$2 ~ /^W[A-Z0-9]+$/ ||