nfsd: fix stripe size reply for the File Layout pNFS server

At a recent testing event I found out that I had misinterpreted
RFC5661 where it describes the stripe size in the File Layout's
nfl_util field. This patch fixes the pNFS File Layout server
so that it returns the correct value to the NFSv4.1/4.2 pNFS
enabled client.

This affects almost no one, since pNFS server configurations
are rare and the extant pNFS aware NFS clients seemed to
function correctly despite the erroneous stripe size.
It *might* be needed for correct behaviour if a recent
Linux client mounts a FreeBSD pNFS server configuration
that is using File Layout (non-mirrored configuration).

MFC after: 2 weeks