Refs #7794
Iff we need to pre-fill segment file ni O_DSYNC mode, we should
drop this for the pre-fill, to avoid issuing flushes until the file
is filled. Done by temporarily closing, re-opening in "normal" mode,
filling, then re-opening.
v2:
* More comment
v3:
* Add missing flush
v4:
* comment
v5:
* Split coroutine and fix into separate patches