Fix for ARC sysctls ignored at runtime

This change leverage module_param_call() to run arc_tuning_update()
immediately after the ARC tunable has been updated as suggested in
cffa8372 code review.

A simple test case is added to the ZFS Test Suite to prevent future
regressions in functionality.

Reviewed-by: Matt Macy <mmacy@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #9487  
Closes #9489

include/sys/zfs_context.h

@@ -198,7 +198,13 @@ extern int aok;
 /*
  * Tunables.
  */
+typedef struct zfs_kernel_param {
+	const char *name;	/* unused stub */
+} zfs_kernel_param_t;
+
 #define	ZFS_MODULE_PARAM(scope_prefix, name_prefix, name, type, perm, desc)
+#define	ZFS_MODULE_PARAM_CALL(scope_prefix, name_prefix, name, setfunc, \
+	getfunc, perm, desc)
 
 /*
  * Exported symbols