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/checkpoint.c b/fs/f2fs/checkpoint.c
index 757b77b..c8516ee 100644
--- a/fs/f2fs/checkpoint.c
+++ b/fs/f2fs/checkpoint.c
@@ -82,6 +82,7 @@
 		return NM_I(sbi)->max_nid / NAT_ENTRY_PER_BLOCK;
 	case META_SIT:
 		return SIT_BLK_CNT(sbi);
+	case META_SSA:
 	case META_CP:
 		return 0;
 	default:
@@ -90,7 +91,7 @@
 }
 
 /*
- * Readahead CP/NAT/SIT pages
+ * Readahead CP/NAT/SIT/SSA pages
  */
 int ra_meta_pages(struct f2fs_sb_info *sbi, int start, int nrpages, int type)
 {
@@ -125,8 +126,9 @@
 				goto out;
 			prev_blk_addr = blk_addr;
 			break;
+		case META_SSA:
 		case META_CP:
-			/* get cp block addr */
+			/* get ssa/cp block addr */
 			blk_addr = blkno;
 			break;
 		default: