Periodically update ARC kstats

FreeBSD needs arc_adjust_zthr to run periodically for kstats to be
updated.  A comment in the code suggests this may have been the
original intent in illumos as well:

https://github.com/openzfs/zfs/blob/c946d5a91329b075fb9bda1ac703a2e85139cf1c/module/zfs/arc.c#L4697-L4700

Create the thread with a 1 second timer.

Reviewed-by: Matt Macy <mmacy@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10371

module/zfs/arc.c

@@ -7360,8 +7360,8 @@ arc_init(void)
 		kstat_install(arc_ksp);
 	}
 
-	arc_adjust_zthr = zthr_create(arc_adjust_cb_check,
-	    arc_adjust_cb, NULL);
+	arc_adjust_zthr = zthr_create_timer(arc_adjust_cb_check,
+	    arc_adjust_cb, NULL, SEC2NSEC(1));
 	arc_reap_zthr = zthr_create_timer(arc_reap_cb_check,
 	    arc_reap_cb, NULL, SEC2NSEC(1));