SUNRPC: Clean up request deferral tracepoints

- Rename these so they are easy to enable and search for as a set
- Move the tracepoints to get a more accurate sense of control flow
- Tracepoints should not fire on xprt shutdown
- Display memory address in case data structure had been corrupted
- Abandon dprintk in these paths

I haven't ever gotten one of these tracepoints to trigger. I wonder
if we should simply remove them.

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