- Hierarchy of state attached to nfs4_client in order to
    simplify state recovery.
    state_owners hang off nfs4_client, whereas state hangs
    off both open_owners and the nfs_inode.
    
  - Model tries to minimize the number of open_owners on
    the server by recycling unused open_owners into a pool.
    
  - NFSv4 state attached to file->private_data. Previously
    this was used by credentials (and still is for NFSv2/v3)
    Abstract out setup/release of struct file and nfs_page
    structure initialization in order to cope with these
    conflicting uses of private_data.