Change the pNFS server so that it does a distributed layout of mirrored
storage files on the DSs instead of strict sets of mirrored DSs.
This actually simplified the code in various places and made doing
migration easy. It also implies that typically only one DS will run
out of space at a time and can be disabled, allowing the service to
continue to run until other DSs run out of space.
It also adds the capability of migration, where a sysadmin can move
data storage files between the DSs.
The system call that does this could be used by a load/storage space
balancer in the future.
Patched versions of nfsd and pnfsdscopymr will be committed soon to
make this work.