Fix race condition with blockdev/BLKRRPART due to lack of fsync

Fix race condition with blockdev/BLKRRPART due to lack of fsync

Quoting dann frazier from Debian's #975015:

| grml2usb autopkgtest frequently fail in Ubuntu's CI.
| [...]
| 2020-11-17 14:14:49,433 Probing device via 'blockdev --rereadpt /dev/loop0'
| blockdev: ioctl error on BLKRRPART: Device or resource busy
| 2020-11-17 14:14:49,452 Execution failed: ("Couldn't execute blockdev on '%s' (install util-linux?)", '/dev/loop0')
|
| I am able to reproduce this on an OpenStack instance with a failure rate of
| 33% (36 failures, 110 passes). My theory is that the sync is not sufficient,
| and that we really need to do a targeted fsync of the file. With the
| attached patch, I've yet to see a failure in 42 iterations.

Thanks: dann frazier <dannf@dannf.org> for bug report and patch
Closes: #975015