Larger metadata clusters can significantly improve writeback performance
on ssd drives with large erasure blocks.  The larger clusters make it
more likely a given IO will completely overwrite the ssd block, so it
doesn't have to do an internal rwm cycle.

On spinning media, lager metadata clusters end up spreading out the
metadata more over time, which makes fsck slower, so we don't want this
to be the default.
Signed-off-by: NChris Mason <chris.mason@oracle.com>