Add support for kernel 4.15

Fixes #260

mod/stateful/timer.c

@@ -1,10 +1,24 @@
 #include "nat64/mod/stateful/timer.h"
 
+#include "nat64/mod/common/linux_version.h"
 #include "nat64/mod/common/xlator.h"
 #include "nat64/mod/stateful/fragment_db.h"
 #include "nat64/mod/stateful/joold.h"
 #include "nat64/mod/stateful/bib/db.h"
 
+/*
+ * TODO We don't cancel the timer much at all; it seems like we should be using
+ * a hrtimer instead.
+ *
+ * 	the kernel has two core timer mechanisms. One of those — the
+ * 	high-resolution timer (or "hrtimer") — subsystem, is focused on
+ * 	near-term events where the timer is expected to run to completion. The
+ * 	other subsystem is just called "kernel timers"; it offers less precision
+ * 	but is more efficient in situations where the timer will probably be
+ * 	canceled before it fires.
+ * 		(Jonathan Corbet, 2017)
+ */
+
 #define TIMER_PERIOD msecs_to_jiffies(2000)
 
 static struct timer_list timer;
@@ -17,7 +31,13 @@ static int clean_state(struct xlator *jool, void *args)
 	return 0;
 }
 
-static void timer_function(unsigned long arg)
+static void timer_function(
+#if LINUX_VERSION_AT_LEAST(4, 15, 0, 9999, 0)
+		struct timer_list *arg
+#else
+		unsigned long arg
+#endif
+		)
 {
 	xlator_foreach(clean_state, NULL);
 	mod_timer(&timer, jiffies + TIMER_PERIOD);
@@ -28,10 +48,14 @@ static void timer_function(unsigned long arg)
  */
 int timer_init(void)
 {
+#if LINUX_VERSION_AT_LEAST(4, 15, 0, 9999, 0)
+	timer_setup(&timer, timer_function, 0);
+#else
 	init_timer(&timer);
 	timer.function = timer_function;
 	timer.expires = 0;
 	timer.data = 0;
+#endif
 	mod_timer(&timer, jiffies + TIMER_PERIOD);
 	return 0;
 }