SUNRPC: Prepare for xdr_stream-style decoding on the server-side

A "permanent" struct xdr_stream is allocated in struct svc_rqst so
that it is usable by all server-side decoders. A per-rqst scratch
buffer is also allocated to handle decoding XDR data items that
cross page boundaries.

To demonstrate how it will be used, add the first call site for the
new svcxdr_init_decode() API.

As an additional part of the overall conversion, add symbolic
constants for successful and failed XDR operations. Returning "0" is
overloaded. Sometimes it means something failed, but sometimes it
means success. To make it more clear when XDR decoding functions
succeed or fail, introduce symbolic constants.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>