SUNRPC: clean up rpc_setup_pipedir()

There is still a little wart or two there: Since we've already got a
vfsmount, we might as well pass that in to rpc_create_client_dir.
Another point is that if we open code __rpc_lookup_path() here, then we can
avoid looking up the entire parent directory path over and over again: it
doesn't change.

Also get rid of rpc_clnt->cl_pathname, since it has no users...

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>