bug#51345: dd with conv=fsync sometimes returns when its writes are still cached

[Sworddragon]

On Knoppix 9.1 with the Linux Kernel 5.10.10-64 x86_64 and GNU Coreutils
8.32 I wanted to overwrite my USB Thumb Drive a few times with random data
via "dd if=/dev/random of=/dev/sdb bs=1M conv=fsync". While it usually
takes ~2+ minutes to perform this action dd returned once after less than
60 seconds which made me a bit curious. On a later attempt dd returned with
this command after ~1 minute again but the LED on my USB Thumb Drive was
still blinking and an immediated executed sync on the terminal blocked for
~1 minute and once it returned the LED on my USB Thumb Drive also stopped
blinking.

Knoppix 9.1 was booted as a live system from a DVD and the only another
persistent storage attached to it was an internal HDD which was already
overwritten the same way via a past booting session.


Unfortunately Linux is not my main operating system anymore so I randomly
encountered this issue on a nearly 1 year old distribution which might be
slightly outdated. But the issue is pretty severe as it can lead to
significant data loss so it is worth at least reporting it (and having the
"bad" behavior to always call sync after dd nonetheless will probably now
continue to stay for quite a while).