Allow the ldiskfs mb_c[123]_threshold parameters to be set persistently. Currently, they are only in /sys/fs/ldiskfs/<dev>/mb_c[123]_threshold and are not configurable via "lctl set_param", sysctl, or other (known to me) tuning mechanism.

While the upstream ext4 developers seem to frown upon mount options, this does provide a mechanism (via /etc/fstab and/or tune2fs -E mount_opts=<opts>) for setting filesystems parameters permanently on a per-filesystem basis.