sunrpc: Change ret code of xdr_stream_decode_opaque_fixed

Since the opaque is fixed in size, the caller already knows how many
bytes were decoded, on success. Thus, xdr_stream_decode_opaque_fixed()
doesn't need to return that value. And, xdr_stream_decode_u32 and _u64
both return zero on success.

This patch simplifies the caller's error checking to avoid potential
integer promotion issues.

Suggested-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Sergey Bashirov <sergeybashirov@gmail.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>