netfilter: extract Passive OS fingerprint infrastructure from xt_osf

Add nf_osf_ttl() and nf_osf_match() into nf_osf.c to prepare for
nf_tables support.

Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/linux/netfilter/nf_osf.h

@@ -0,0 +1,27 @@
+#include <uapi/linux/netfilter/nf_osf.h>
+
+/* Initial window size option state machine: multiple of mss, mtu or
+ * plain numeric value. Can also be made as plain numeric value which
+ * is not a multiple of specified value.
+ */
+enum nf_osf_window_size_options {
+	OSF_WSS_PLAIN   = 0,
+	OSF_WSS_MSS,
+	OSF_WSS_MTU,
+	OSF_WSS_MODULO,
+	OSF_WSS_MAX,
+};
+
+enum osf_fmatch_states {
+	/* Packet does not match the fingerprint */
+	FMATCH_WRONG = 0,
+	/* Packet matches the fingerprint */
+	FMATCH_OK,
+	/* Options do not match the fingerprint, but header does */
+	FMATCH_OPT_WRONG,
+};
+
+bool nf_osf_match(const struct sk_buff *skb, u_int8_t family,
+		  int hooknum, struct net_device *in, struct net_device *out,
+		  const struct nf_osf_info *info, struct net *net,
+		  const struct list_head *nf_osf_fingers);