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.c

@@ -0,0 +1,18 @@
+/* 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.
+ */
+
+#include <stdint.h>
+
+#include "flow.h"
+
+const char *flow_type_str[] = {
+	[FLOW_TYPE_NONE]	= "<none>",
+	[FLOW_TCP]		= "TCP connection",
+	[FLOW_TCP_SPLICE]	= "TCP connection (spliced)",
+};
+static_assert(ARRAY_SIZE(flow_type_str) == FLOW_NUM_TYPES,
+	      "flow_type_str[] doesn't match enum flow_type");