md: drop null test before destroy functions

Remove unneeded NULL test.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@ expression x; @@
-if (x != NULL)
  \(kmem_cache_destroy\|mempool_destroy\|dma_pool_destroy\)(x);
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: NeilBrown <neilb@suse.com>

drivers/md/multipath.c

@@ -470,8 +470,7 @@ static int multipath_run (struct mddev *mddev)
 	return 0;
 
 out_free_conf:
-	if (conf->pool)
-		mempool_destroy(conf->pool);
+	mempool_destroy(conf->pool);
 	kfree(conf->multipaths);
 	kfree(conf);
 	mddev->private = NULL;