Prepare nilfs for vcache:
- Calling getnewvnode() with "mp == NULL" is wrong.  Stop attaching a
  vnode to system file nodes and change nilfs_bread() to translate
  the block address and then uyse the device for the read.
- Move the vnode initialisation to nilfs_get_node() and use
  nilfs_get_node_raw() to initialise the nilfs node only.
- Same for nilfs_reclaim() versus nilfs_dispose_node().
- Change nilfs_get_node() to return an unlocked vnode instead of
  a nilfs node with locked vnode.  Adapt nilfs_lookup() and nilfs_root().
- Don't treat unsupported node types (blk, chr ...) as regular,
  return ENXIO instead.
- Fix nilfs_getattr() to mask the mode with ALLPERMS.
- Destroy sync_cv before free.
Change nilfs to vcache.