Only allocate the nfslayouthash table when a pNFS service is started the
first time. This allows the allocation to be quite large, since pNFS
MDS servers will normally be dedicated fairly large servers.
Replace vfs.nfsd.layouthashsize with vfs.nfsd.layouthighwater, so that
both the maximum # of layouts in the server and the size of the hash table
can both be tuned with this.