So I looked at this a bit, and doing the right thing (TM) will be a little invase and thus not for 5.5. But the 5.5. queue already has a patch from Mikulas Patocka: 8b2ded1c94c ("block: don't warn when doing fsync on read-only devices") which should deal with this issue, and in fact I can't trigger the WARN_ON with Jens' latest tree.