LU-17423 ldiskfs: pass NULL to ext4_dir_rec_len for dot and dotdot

LU-17423 ldiskfs: pass NULL to ext4_dir_rec_len for dot and dotdot

For '.' and '..' we should pass NULL to EXT4_DIR_ENTRY_LEN/
EXT4_DIR_REC_LEN/ext4_dir_rec_len() as those entries do not
use extra fields to store the extra hash for casefolded+fscrypt
case.

This has no impact yet as we do not support casefolded+fscrypt
right now.

Change-Id: I6df56aefdf6440e4c03088fec7a4d38a523cf8dc
Test-Parameters: trivial
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54724
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>