As OST sizes become larger, exceeding 1PiB in some cases, it may be necessary to have single OST object directories with more than 10M objects in them in order to hold up to 4B objects.

In many cases, the MDS will start to use more FID Sequence numbers since LU-11912 landed, but large_dir is already enabled automatically on OSTs at mount time since LU-11546, so should also be set at format time to avoid a console message.