bug#80106: Don't sort dirents (in du) for Lustre

The gnulib function dirent_inode_sort_may_be_useful() should return
false for Lustre (i.e. #define S_MAGIC_LUSTRE 0x0BD00BD0 as seen
in lustre/include/uapi/linux/lustre/lustre_user.h in the Lustre source
tree as LL_SUPER_MAGIC [1]). Sorting dirents negatively impacts du
performance on Lustre because it interferes with Lustre's ability to
prefetch file metadata (via statahead).

For context, Lustre is an open-source (GPLv2) out-of-tree Linux filesystem
commonly used for HPC applications.

Let me know if you need anymore context. Thanks!

Tim Day

[1] https://git.whamcloud.com/?p=fs/lustre-release.git;a=blob;f=lustre/include/uapi/linux/lustre/lustre_user.h