f2fs: readahead contiguous SSA blocks for f2fs_gc

If there are multi segments in one section, we will read those SSA blocks which
have contiguous address one by one in f2fs_gc. It may lost performance, let's
read ahead SSA blocks by merge multi read request.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 6f88191..bd6666e 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -89,12 +89,13 @@
 };
 
 /*
- * For CP/NAT/SIT readahead
+ * For CP/NAT/SIT/SSA readahead
  */
 enum {
 	META_CP,
 	META_NAT,
-	META_SIT
+	META_SIT,
+	META_SSA
 };
 
 /* for the list of orphan inodes */