osd: make filestore_blackhole works

filestore_blackhole is useful for performance test, but we should make
it works for client, so the patch queue ondisk/onreadable/
onreadable_sync directly into finisher.

The patch also set the dout level of filestore_blackhole to 1, so it
will not hurt the performance.

Signed-off-by: Dong Yuan <yuandong1222@gmail.com>