LU-16821 llite: report 1MiB directory blocksize

LU-16821 llite: report 1MiB directory blocksize

Report st_blksize=1048576 for directories so that glibc readdir()
will allocate a larger buffer to match the MDS_READDIR size
and reduce the number of syscalls for large dirs.

Signed-off-by: Andreas Dilger <adilger@dilger.ca>
Change-Id: If64057c20ecc35194c319d2a88c3036f12c41ed5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50960
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>