kernel timer API rework

In `config/kernel-timer.m4` refactor slightly to check more generally
for the new `timer_setup()` APIs, but also check the callback signature
because some kernels (notably 4.14) have the new `timer_setup()` API but
use the old callback signature. Also add a check for a `flags` member in
`struct timer_list`, which was added in 4.1-rc8.

Add compatibility shims to `include/spl/sys/timer.h` to allow using the
new timer APIs with the only two caveats being that the callback
argument type must be declared as `spl_timer_list_t` and an explicit
assignment is required to get the timer variable for the `timer_of()`
macro. So the callback would look like this:

```c
__cv_wakeup(spl_timer_list_t t)
{
        struct timer_list *tmr = (struct timer_list *)t;
	struct thing *parent = from_timer(parent, tmr,
		parent_timer_field);
	... /* do stuff with parent */
```

Make some minor changes to `spl-condvar.c` and `spl-taskq.c` to use the
new timer APIs instead of conditional code.

Reviewed-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rafael Kitover <rkitover@gmail.com>
Closes #8647

include/spl/sys/timer.h

@@ -72,4 +72,29 @@ usleep_range(unsigned long min, unsigned long max)
 #define	USEC_TO_TICK(us)		usecs_to_jiffies(us)
 #define	NSEC_TO_TICK(ns)		usecs_to_jiffies(ns / NSEC_PER_USEC)
 
+#ifndef from_timer
+#define	from_timer(var, timer, timer_field) \
+	container_of(timer, typeof(*var), timer_field)
+#endif
+
+#ifdef HAVE_KERNEL_TIMER_FUNCTION_TIMER_LIST
+typedef struct timer_list *spl_timer_list_t;
+#else
+typedef unsigned long spl_timer_list_t;
+#endif
+
+#ifndef HAVE_KERNEL_TIMER_SETUP
+
+static inline void
+timer_setup(struct timer_list *timer, void (*func)(spl_timer_list_t), u32 fl)
+{
+#ifdef HAVE_KERNEL_TIMER_LIST_FLAGS
+	(timer)->flags = fl;
+#endif
+	init_timer(timer);
+	setup_timer(timer, func, (spl_timer_list_t)(timer));
+}
+
+#endif /* HAVE_KERNEL_TIMER_SETUP */
+
 #endif  /* _SPL_TIMER_H */