dm-crypt: fix warning in shutdown path

The counter for the number of allocated pages includes pages in the
mempool's reserve, so checking that the number of allocated pages is 0
needs to happen after we exit the mempool.

Fixes: 6f1c819c ("dm: convert to bioset_init()/mempool_init()")
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Reported-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Mike Snitzer <snitzer@redhat.com>

Fixed to always just use percpu_counter_sum()
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/dm-crypt.c

@@ -2221,14 +2221,13 @@ static void crypt_dtr(struct dm_target *ti)
 
 	bioset_exit(&cc->bs);
 
-	if (mempool_initialized(&cc->page_pool))
-		WARN_ON(percpu_counter_sum(&cc->n_allocated_pages) != 0);
-	percpu_counter_destroy(&cc->n_allocated_pages);
-
 	mempool_exit(&cc->page_pool);
 	mempool_exit(&cc->req_pool);
 	mempool_exit(&cc->tag_pool);
 
+	WARN_ON(percpu_counter_sum(&cc->n_allocated_pages) != 0);
+	percpu_counter_destroy(&cc->n_allocated_pages);
+
 	if (cc->iv_gen_ops && cc->iv_gen_ops->dtr)
 		cc->iv_gen_ops->dtr(cc);