Fix the FreeBSD userspace build (#15716)

- Mark some parameters to zpool_power*() as unused.
- Add a stub zpool_disk_wait().

Fixes: a9520e6 ("zpool: Add slot power control, print power status")

Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>

cmd/zpool/os/freebsd/zpool_vdev_os.c

@@ -120,13 +120,20 @@ after_zpool_upgrade(zpool_handle_t *zhp)
 int
 zpool_power_current_state(zpool_handle_t *zhp, char *vdev)
 {
+
+	(void) zhp;
+	(void) vdev;
 	/* Enclosure slot power not supported on FreeBSD yet */
 	return (-1);
 }
 
 int
 zpool_power(zpool_handle_t *zhp, char *vdev, boolean_t turn_on)
 {
+
+	(void) zhp;
+	(void) vdev;
+	(void) turn_on;
 	/* Enclosure slot power not supported on FreeBSD yet */
 	return (ENOTSUP);
 }

lib/libzutil/os/freebsd/zutil_import_os.c

@@ -261,3 +261,11 @@ void
 update_vdevs_config_dev_sysfs_path(nvlist_t *config)
 {
 }
+
+int
+zpool_disk_wait(const char *path)
+{
+
+	(void) path;
+	return (ENOTSUP);
+}