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