FreeBSD: notify userspace when a vdev is removed

This is needed for zfsd to autoreplace vdevs.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11260

module/zfs/spa.c

@@ -7903,6 +7903,9 @@ spa_async_remove(spa_t *spa, vdev_t *vd)
 		vd->vdev_stat.vs_checksum_errors = 0;
 
 		vdev_state_dirty(vd->vdev_top);
+
+		/* Tell userspace that the vdev is gone. */
+		zfs_post_remove(spa, vd);
 	}
 
 	for (int c = 0; c < vd->vdev_children; c++)