f2fs: avoid victim selection from previous victim section

When f2fs chooses GC victim in large section & LFS mode,
next_victim_seg[gc_type] is referenced first. After segment is freed,
next_victim_seg[gc_type] has the next segment number.
However, next_victim_seg[gc_type] still has the last segment number
even after the last segment of section is freed. In this case, when f2fs
chooses a victim for the next GC round, the last segment of previous victim
section is chosen as a victim.

Initialize next_victim_seg[gc_type] to NULL_SEGNO for the last segment in
large section.

Fixes: e3080b01 ("f2fs: support subsectional garbage collection")
Signed-off-by: Yonggil Song <yonggil.song@samsung.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/gc.c

@@ -1749,8 +1749,9 @@ static int do_garbage_collect(struct f2fs_sb_info *sbi,
 				get_valid_blocks(sbi, segno, false) == 0)
 			seg_freed++;
 
-		if (__is_large_section(sbi) && segno + 1 < end_segno)
-			sbi->next_victim_seg[gc_type] = segno + 1;
+		if (__is_large_section(sbi))
+			sbi->next_victim_seg[gc_type] =
+				(segno + 1 < end_segno) ? segno + 1 : NULL_SEGNO;
 skip:
 		f2fs_put_page(sum_page, 0);
 	}