xdr_reserve_space - Reserve buffer space for sending
uint32_t * xdr_reserve_space (struct xdr_stream * xdr, size_t nbytes);
xdr pointer to xdr_stream nbytes number of bytes to reserve
Checks that we have enough buffer space to encode ’nbytes’ more bytes of data. If so, update the total xdr_buf length, and adjust the length of the current kvec.