[LogFS] Prevent mempool_destroy NULL pointer dereference

It would probably be better to just accept NULL pointers in
mempool_destroy().  But for the current -rc series let's keep things
simple.

This patch was lost in the cracks for a while.
Kevin Cernekee <cernekee@gmail.com> had to rediscover the problem and
send a similar patch because of it. :(

Signed-off-by: Joern Engel <joern@logfs.org>
diff --git a/fs/logfs/super.c b/fs/logfs/super.c
index d6e1f4f..d4531eb 100644
--- a/fs/logfs/super.c
+++ b/fs/logfs/super.c
@@ -517,8 +517,8 @@
 	if (super->s_erase_page)
 		__free_page(super->s_erase_page);
 	super->s_devops->put_device(sb);
-	mempool_destroy(super->s_btree_pool);
-	mempool_destroy(super->s_alias_pool);
+	logfs_mempool_destroy(super->s_btree_pool);
+	logfs_mempool_destroy(super->s_alias_pool);
 	kfree(super);
 	log_super("LogFS: Finished unmounting\n");
 }