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-cache-target.c

@@ -2309,8 +2309,7 @@ static void destroy(struct cache *cache)
 {
 	unsigned i;
 
-	if (cache->migration_pool)
-		mempool_destroy(cache->migration_pool);
+	mempool_destroy(cache->migration_pool);
 
 	if (cache->all_io_ds)
 		dm_deferred_set_destroy(cache->all_io_ds);