nfsd: tabulate nfs4 xdr encoding functions

In preparation for minorversion 1

All encoders now return an nfserr status (typically their
nfserr argument).  Unsupported ops go through nfsd4_encode_operation
too, so use nfsd4_encode_noop to encode nothing for their reply body.
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>