Here is the a pre-patch in the attempt to get rid of 'struct
nfs4_compound', and the associated horrible union in 'struct
nfs4_op'.

It splits out the fields that are meant to do buffer overflow checking
and iovec adjusting on the XDR received/sent data. It moves support
for that nto the dedicated structure 'xdr_stream', and the associated
functions 'xdr_reserve_space()', 'xdr_inline_decode()'.

The patch also expands out the all macros ENCODE_HEAD, ENCODE_TAIL,
ADJUST_ARGS and DECODE_HEAD, as well as most of the DECODE_TAILs.