busybox: revert dd fsync change which makes no sense

[MilhouseVH]

This change (introduced in 1.27.0) makes no sense - if anyone wanted to perform a single sync they would do that with dd && sync without changing the entire behaviour of conv=fsync.

Following this change in busybox, the upgrade progress indicators are useless as dd bs=1M conv=fsync no longer performs a sync every 1MB - there are now long delays where there is no progress, and then the progress jumps to 50% and finally 100% as the entire transfer is synced at the end (rather than at regular intervals during the transfer prior to busybox 1.27.0).

I've opened a bug: https://bugs.busybox.net/show_bug.cgi?id=10191

Apologies for not catching this sooner, I had assumed my SD card was on the fritz...!

[vpeter4]

For a workaround you could make a loop around dd and wrote 1MB at a time :)

[MilhouseVH]

Yeah... that would be niiice! :)

Well, it looks like this won't be changing (see reply on bug) so I guess we'll simply continue to revert this change.

We could run sync every second in the monitoring thread but... meh. That's Plan B should reverting the change bite the dust (which is unlikely, as it's so trivial). Although if anyone prefers Plan B, please shout...