LU-18518 ldiskfs: fix LC_SRC_NFS_FILLDIR_USE_CTX check

LU-18518 ldiskfs: fix LC_SRC_NFS_FILLDIR_USE_CTX check

When compiled with clang, these checks always failed, resulting
in lustre device mount being stuck.

Signed-off-by: Sohei Koyama <skoyama@ddn.com>
Change-Id: I7bac750700b791046050964bdcd59c2f05d7d986
Test-Parameters: trivial
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57333
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>