basic/linux: Copy netfilter.h to the source tree

This fixes build with old toolchains prior to Linux < 4.2 which do not
have a definition for NFPROTO_NETDEV.

src/basic/linux/netfilter.h

@@ -0,0 +1,76 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef __LINUX_NETFILTER_H
+#define __LINUX_NETFILTER_H
+
+#include <linux/types.h>
+
+#include <linux/in.h>
+#include <linux/in6.h>
+
+/* Responses from hook functions. */
+#define NF_DROP 0
+#define NF_ACCEPT 1
+#define NF_STOLEN 2
+#define NF_QUEUE 3
+#define NF_REPEAT 4
+#define NF_STOP 5	/* Deprecated, for userspace nf_queue compatibility. */
+#define NF_MAX_VERDICT NF_STOP
+
+/* we overload the higher bits for encoding auxiliary data such as the queue
+ * number or errno values. Not nice, but better than additional function
+ * arguments. */
+#define NF_VERDICT_MASK 0x000000ff
+
+/* extra verdict flags have mask 0x0000ff00 */
+#define NF_VERDICT_FLAG_QUEUE_BYPASS	0x00008000
+
+/* queue number (NF_QUEUE) or errno (NF_DROP) */
+#define NF_VERDICT_QMASK 0xffff0000
+#define NF_VERDICT_QBITS 16
+
+#define NF_QUEUE_NR(x) ((((x) << 16) & NF_VERDICT_QMASK) | NF_QUEUE)
+
+#define NF_DROP_ERR(x) (((-x) << 16) | NF_DROP)
+
+/* only for userspace compatibility */
+
+/* NF_VERDICT_BITS should be 8 now, but userspace might break if this changes */
+#define NF_VERDICT_BITS 16
+
+enum nf_inet_hooks {
+	NF_INET_PRE_ROUTING,
+	NF_INET_LOCAL_IN,
+	NF_INET_FORWARD,
+	NF_INET_LOCAL_OUT,
+	NF_INET_POST_ROUTING,
+	NF_INET_NUMHOOKS,
+	NF_INET_INGRESS = NF_INET_NUMHOOKS,
+};
+
+enum nf_dev_hooks {
+	NF_NETDEV_INGRESS,
+	NF_NETDEV_EGRESS,
+	NF_NETDEV_NUMHOOKS
+};
+
+enum {
+	NFPROTO_UNSPEC =  0,
+	NFPROTO_INET   =  1,
+	NFPROTO_IPV4   =  2,
+	NFPROTO_ARP    =  3,
+	NFPROTO_NETDEV =  5,
+	NFPROTO_BRIDGE =  7,
+	NFPROTO_IPV6   = 10,
+	NFPROTO_DECNET = 12,
+	NFPROTO_NUMPROTO,
+};
+
+union nf_inet_addr {
+	__u32		all[4];
+	__be32		ip;
+	__be32		ip6[4];
+	struct in_addr	in;
+	struct in6_addr	in6;
+};
+
+#endif /* __LINUX_NETFILTER_H */