NFSD: Clean up nfsd4_encode_replay()

Replace open-coded encoding logic with the use of conventional XDR
utility functions. Add a tracepoint to make replays observable in
field troubleshooting situations.

The WARN_ON is removed. A stack trace is of little use, as there is
only one call site for nfsd4_encode_replay(), and a buffer length
shortage here is unlikely.

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