inpricise checkpoint stats

test=# set log_statement_stats = on;
SET
test=# checkpoint;
LOG:  QUERY STATISTICS
DETAIL:  ! system usage stats:
        !       0.100725 elapsed 0.000000 user 0.000000 system sec
        !       [0.000000 user 0.001999 sys total]
        !       0/0 [0/0] filesystem blocks in/out
        !       0/0 [0/319] page faults/reclaims, 0 [0] swaps
        !       0 [0] signals rcvd, 0/0 [0/0] messages rcvd/sent
        !       1/1 [8/6] voluntary/involuntary context switches
        ! buffer usage stats:
        !       Shared blocks:          0 read,          0 written, buffer
hit rate = 0.00%
        !       Local  blocks:          0 read,          0 written, buffer
hit rate = 0.00%
        !       Direct blocks:          0 read,          0 written
CHECKPOINT

The problematic line is "0 written" -- this is because we bgwriter is
actually the command exeuctor. So in our process, we got nothing. But maybe
not deserved a fix ...

Regards,
Qingqing