flow, tcp: Generalise connection types

Currently TCP connections use a 1-bit selector, 'spliced', to determine the
rest of the contents of the structure.  We want to generalise the TCP
connection table to other types of flows in other protocols.  Make a start
on this by replacing the tcp_conn_common structure with a new flow_common
structure with an enum rather than a simple boolean indicating the type of
flow.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

flow.h

@@ -0,0 +1,36 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later
+ * Copyright Red Hat
+ * Author: David Gibson <david@gibson.dropbear.id.au>
+ *
+ * Tracking for logical "flows" of packets.
+ */
+#ifndef FLOW_H
+#define FLOW_H
+
+/**
+ * enum flow_type - Different types of packet flows we track
+ */
+enum flow_type {
+	/* Represents an invalid or unused flow */
+	FLOW_TYPE_NONE = 0,
+	/* A TCP connection between a socket and tap interface */
+	FLOW_TCP,
+	/* A TCP connection between a host socket and ns socket */
+	FLOW_TCP_SPLICE,
+
+	FLOW_NUM_TYPES,
+};
+
+extern const char *flow_type_str[];
+#define FLOW_TYPE(f)							\
+        ((f)->type < FLOW_NUM_TYPES ? flow_type_str[(f)->type] : "?")
+
+/**
+ * struct flow_common - Common fields for packet flows
+ * @type:	Type of packet flow
+ */
+struct flow_common {
+	uint8_t		type;
+};
+
+#endif /* FLOW_H */