dm: drop NULL test before kmem_cache_destroy() and mempool_destroy()

Remove DM's unneeded NULL tests before calling these destroy functions,
now that they check for NULL, thanks to these v4.3 commits:
3942d2991 ("mm/slab_common: allow NULL cache pointer in kmem_cache_destroy()")
4e3ca3e03 ("mm/mempool: allow NULL `pool' pointer in mempool_destroy()")

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: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-region-hash.c

@@ -249,9 +249,7 @@ void dm_region_hash_destroy(struct dm_region_hash *rh)
 	if (rh->log)
 		dm_dirty_log_destroy(rh->log);
 
-	if (rh->region_pool)
-		mempool_destroy(rh->region_pool);
-
+	mempool_destroy(rh->region_pool);
 	vfree(rh->buckets);
 	kfree(rh);
 }