unix: add generic netlink types and constants on Linux

Generic netlink is a generalized netlink family which enables a series
of different communication channels to be multiplexed on a single
netlink family.

For more information, please see: https://lwn.net/Articles/208755/.

Fixes golang/go#21682.

Change-Id: I2095150e06471f4a0e1322fba669573aa8f0a9b1
Reviewed-on: https://go-review.googlesource.com/59931
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/mkerrors.sh

@@ -182,6 +182,7 @@ struct ltchars {
 #include <linux/can.h>
 #include <linux/vm_sockets.h>
 #include <linux/taskstats.h>
+#include <linux/genetlink.h>
 #include <net/route.h>
 #include <asm/termbits.h>
 
@@ -415,6 +416,7 @@ ccflags="$@"
 		$2 ~ /^SPLICE_/ ||
 		$2 ~ /^(VM|VMADDR)_/ ||
 		$2 ~ /^(TASKSTATS|TS)_/ ||
+		$2 ~ /^GENL_/ ||
 		$2 ~ /^XATTR_(CREATE|REPLACE)/ ||
 		$2 !~ "WMESGLEN" &&
 		$2 ~ /^W[A-Z0-9]+$/ ||