SUNRPC: skip clients with program without PipeFS entries

1) This is sane.
2) Otherwise there will be soft lockup:

do {
	rpc_get_client_for_event (clnt->cl_dentry == NULL ==> choose)
	__rpc_pipefs_event (clnt->cl_program->pipe_dir_name == NULL ==> return)
} while (1)

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