dm mpath: fix support for loading scsi_dh modules during table load

The ability to have multipath dynamically attach a scsi_dh, that the user
specified in the multipath table, was broken by commit e8f74a0f ("dm
mpath: eliminate need to use scsi_device_from_queue").

Restore the ability to load, and attach, a particular scsi_dh module if
one is specified (as noticed by checking m->hw_handler_name).

Fixes: e8f74a0f ("dm mpath: eliminate need to use scsi_device_from_queue")
Reported-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-mpath.c

@@ -887,7 +887,7 @@ static struct pgpath *parse_path(struct dm_arg_set *as, struct path_selector *ps
 
 	q = bdev_get_queue(p->path.dev->bdev);
 	attached_handler_name = scsi_dh_attached_handler_name(q, GFP_KERNEL);
-	if (attached_handler_name) {
+	if (attached_handler_name || m->hw_handler_name) {
 		INIT_DELAYED_WORK(&p->activate_path, activate_path_work);
 		r = setup_scsi_dh(p->path.dev->bdev, m, attached_handler_name, &ti->error);
 		if (r) {