nfsd: do_probe_callback should not clear rpc stats

Now that cb_stats are static (since commit
ff7d9756)
there's no need to clear them.

Initially I thought it might make sense to do
that every callback probing but since the stats
are per-program and they are shared between possibly
several client callback instances, zeroing them out
seems like the wrong thing to do.

Note that that commit also introduced a bug
since stats.program is also being cleared in the process
and it is not restored after the memset as it used to be.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>