NFS: remove RPC PipeFS mount point references from NFS cache routines

This is a cleanup patch. We don't need this reference anymore, because DNS
resolver cache now creates it's dentries in per-net operations and on PipeFS
mount/umount notification.
Note that nfs_cache_register_net() now returns 0 instead of -ENOENT in case of
PiepFS superblock absence. This is ok, Dns resolver cache will be regestered on
PipeFS mount event.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>