Use fcntl(..., F_FULLSYNC) if available

Use fcntl(..., F_FULLSYNC) if available

Some operating systems don't perform a data integrity flush when
fsync() is done, but provide fcntl(fd, F_FULLSYNC) to provide that kind
of guarantee.

To ensure that comparisons between operating systems is fair, use
fcntl() to do a proper sync if available.

Signed-off-by: Jens Axboe <axboe@kernel.dk>