f2fs: add a slab cache entry for small discards

This patch adds a slab cache entry for small discards.

Each entry consists of:

struct discard_entry {
	struct list_head list;	/* list head */
	block_t blkaddr;	/* block address to be discarded */
	int len;		/* # of consecutive blocks of the discard */
};

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index bafff72..e9aa3f7 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -1078,9 +1078,12 @@
 	err = create_node_manager_caches();
 	if (err)
 		goto free_inodecache;
-	err = create_gc_caches();
+	err = create_segment_manager_caches();
 	if (err)
 		goto free_node_manager_caches;
+	err = create_gc_caches();
+	if (err)
+		goto free_segment_manager_caches;
 	err = create_checkpoint_caches();
 	if (err)
 		goto free_gc_caches;
@@ -1102,6 +1105,8 @@
 	destroy_checkpoint_caches();
 free_gc_caches:
 	destroy_gc_caches();
+free_segment_manager_caches:
+	destroy_segment_manager_caches();
 free_node_manager_caches:
 	destroy_node_manager_caches();
 free_inodecache: