SUNRPC: Add generic helpers for xdr_stream encode/decode

Add some generic helpers for encoding/decoding opaque structures and
basic u32/u64.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>