mkfs.ubifs: Fix incorrect dir size calculation in encryption scenario

mkfs.ubifs: Fix incorrect dir size calculation in encryption scenario

The size of directory should be the total length of encrypted entry name,
otherwise it could trigger errors while checking filesystem:
 dbg_check_filesystem [ubifs]: directory inode 89 size is 352, but
 calculated size is 400

Fixes: 4c55918dd747d ("Implement filename encryption")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>