SUNPRC: cleanup RPC PipeFS pipes upcall interface

RPC pipe upcall doesn't requires only private pipe data. Thus RPC inode
references in this code can be removed.

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