SUNRPC: Move XDR data type size macros

Clean up: To make subsequent patches cleaner, move the XDR data type
size macros to the top of the file (similar to nfs4xdr.c) first.

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