SUNRPC: define xdr_decode_opaque_fixed

A helper for decoding a fixed length opaque value.
Returns a pointer to the next item in the xdr stream.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: Fred Isaman <iisaman@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>