f2fs: add quick mode of checkpoint=disable for QA

This mode returns mount() quickly with EAGAIN. We can trigger this by
shutdown(F2FS_GOING_DOWN_NEED_FSCK).

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -868,6 +868,9 @@ int f2fs_disable_cp_again(struct f2fs_sb_info *sbi)
 
 	if (holes[DATA] > ovp || holes[NODE] > ovp)
 		return -EAGAIN;
+	if (is_sbi_flag_set(sbi, SBI_CP_DISABLED_QUICK) &&
+		dirty_segments(sbi) > overprovision_segments(sbi))
+		return -EAGAIN;
 	return 0;
 }