unix: implement Linux AF_XDP sockets

Provides access to the Linux AF_XDP socket family.  For more
information on AF_XDP, see:
https://www.kernel.org/doc/html/latest/networking/af_xdp.html.

Change-Id: I7efb82c4a67da3e15932e3c0d882f0bad7ba16ef
Reviewed-on: https://go-review.googlesource.com/136695
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/mkerrors.sh

@@ -216,6 +216,7 @@ struct ltchars {
 #include <linux/watchdog.h>
 #include <linux/hdreg.h>
 #include <linux/rtc.h>
+#include <linux/if_xdp.h>
 #include <mtd/ubi-user.h>
 #include <net/route.h>
 #include <asm/termbits.h>
@@ -246,6 +247,16 @@ struct ltchars {
 #define FS_KEY_DESC_PREFIX              "fscrypt:"
 #define FS_KEY_DESC_PREFIX_SIZE         8
 #define FS_MAX_KEY_SIZE                 64
+
+// XDP socket constants do not appear to be picked up otherwise.
+// Copied from samples/bpf/xdpsock_user.c.
+#ifndef SOL_XDP
+#define SOL_XDP 283
+#endif
+
+#ifndef AF_XDP
+#define AF_XDP 44
+#endif
 '
 
 includes_NetBSD='
@@ -476,6 +487,7 @@ ccflags="$@"
 		$2 ~ /^FSOPT_/ ||
 		$2 ~ /^WDIOC_/ ||
 		$2 ~ /^NFN/ ||
+		$2 ~ /^XDP_/ ||
 		$2 ~ /^(HDIO|WIN|SMART)_/ ||
 		$2 !~ "WMESGLEN" &&
 		$2 ~ /^W[A-Z0-9]+$/ ||