f2fs: add migration count iff migration happens

If first segment is empty and migration_granularity is 1, we can't move this
at all.
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/gc.c

@@ -1240,12 +1240,12 @@ static int do_garbage_collect(struct f2fs_sb_info *sbi,
 							segno, gc_type);
 
 		stat_inc_seg_count(sbi, type, gc_type);
+		migrated++;
 
 freed:
 		if (gc_type == FG_GC &&
 				get_valid_blocks(sbi, segno, false) == 0)
 			seg_freed++;
-		migrated++;
 
 		if (__is_large_section(sbi) && segno + 1 < end_segno)
 			sbi->next_victim_seg[gc_type] = segno + 1;