dm mpath: fix test for reinstate_path

Fix test for reinstate_path method before attempting to use it.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: Julia Lawall <julia@diku.dk>

drivers/md/dm-mpath.c

diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index 230d789..fea966d 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -871,7 +871,7 @@
 	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;