f2fs: fix start segno of large section

get_ckpt_valid_blocks() checks valid ckpt blocks in current section.
It counts all vblocks from the first to the last segment in the
large section. However, START_SEGNO() is used to get the first segno
in an SIT block. This patch fixes that to get the correct start segno.

Fixes: 61461fc9 ("f2fs: fix to avoid touching checkpointed data in get_victim()")
Signed-off-by: Sheng Yong <shengyong@oppo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.h

@@ -347,7 +347,8 @@ static inline unsigned int get_ckpt_valid_blocks(struct f2fs_sb_info *sbi,
 				unsigned int segno, bool use_section)
 {
 	if (use_section && __is_large_section(sbi)) {
-		unsigned int start_segno = START_SEGNO(segno);
+		unsigned int secno = GET_SEC_FROM_SEG(sbi, segno);
+		unsigned int start_segno = GET_SEG_FROM_SEC(sbi, secno);
 		unsigned int blocks = 0;
 		int i;