ubifs: rename_whiteout: correct old_dir size computing

When renaming the whiteout file, the old whiteout file is not deleted.
Therefore, we add the old dentry size to the old dir like XFS.
Otherwise, an error may be reported due to `fscki->calc_sz != fscki->size`
in check_indes.

Fixes: 9e0a1fff

 ("ubifs: Implement RENAME_WHITEOUT")
Reported-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>