f2fs: missing kmem_cache_destroy for discard_entry

insmod f2fs.ko is failed after insmod and rmmod firstly.

$ sudo insmod fs/f2fs/f2fs.ko
insmod: error inserting 'fs/f2fs/f2fs.ko': -1 Cannot allocate memory

-- dmesg --
kmem_cache_sanity_check (free_nid): Cache name already exists.
Signed-off-by: NChangman Lee <cm224.lee@samsung.com>
Signed-off-by: NJaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/super.c

@@ -1140,6 +1140,7 @@ static void __exit exit_f2fs_fs(void)
 	unregister_filesystem(&f2fs_fs_type);
 	destroy_checkpoint_caches();
 	destroy_gc_caches();
+	destroy_segment_manager_caches();
 	destroy_node_manager_caches();
 	destroy_inodecache();
 	kset_unregister(f2fs_kset);