flow, tcp: Add logging helpers for connection related messages

Most of the messages logged by the TCP code (be they errors, debug or
trace messages) are related to a specific connection / flow.  We're fairly
consistent about prefixing these with the type of connection and the
connection / flow index.  However there are a few places where we put the
index later in the message or omit it entirely.  The template with the
prefix is also a little bulky to carry around for every message,
particularly for spliced connections.

To help keep this consistent, introduce some helpers to log messages
linked to a specific flow.  It takes the flow as a parameter and adds a
uniform prefix to each message.  This makes things slightly neater now, but
more importantly will help keep formatting consistent as we add more things
to the flow table.

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

flow.c

@@ -66,3 +66,21 @@ void flow_table_compact(struct ctx *c, union flow *hole)
 
 	memset(from, 0, sizeof(*from));
 }
+
+/** flow_log_ - Log flow-related message
+ * @f:		flow the message is related to
+ * @pri:	Log priority
+ * @fmt:	Format string
+ * @...:	printf-arguments
+ */
+void flow_log_(const struct flow_common *f, int pri, const char *fmt, ...)
+{
+	char msg[BUFSIZ];
+	va_list args;
+
+	va_start(args, fmt);
+	(void)vsnprintf(msg, sizeof(msg), fmt, args);
+	va_end(args);
+
+	logmsg(pri, "Flow %u (%s): %s", flow_idx(f), FLOW_TYPE(f), msg);
+}