rbd: 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);
  x = NULL;
-}
// </smpl>
Signed-off-by: NJulia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

drivers/block/rbd.c

@@ -5663,10 +5663,8 @@ static int rbd_slab_init(void)
 	if (rbd_segment_name_cache)
 		return 0;
 out_err:
-	if (rbd_obj_request_cache) {
-		kmem_cache_destroy(rbd_obj_request_cache);
-		rbd_obj_request_cache = NULL;
-	}
+	kmem_cache_destroy(rbd_obj_request_cache);
+	rbd_obj_request_cache = NULL;
 
 	kmem_cache_destroy(rbd_img_request_cache);
 	rbd_img_request_cache = NULL;