dm mpath: fix test for reinstate_path

Fix test for reinstate_path method before attempting to use it.

Signed-off-by: Alasdair G Kergon <[e-mail]>
Cc: Julia Lawall <[e-mail]>

drivers/md/dm-mpath.c

···
 	if (pgpath->path.is_active)
 		goto out;
 
-	if (!pgpath->pg->ps.type) {
+	if (!pgpath->pg->ps.type->reinstate_path) {
 		DMWARN("Reinstate path not supported by path selector %s",
 		       pgpath->pg->ps.type->name);
 		r = -EINVAL;