dissect: ext4 and loopback files are unimpressed by read-only access

Even if we set up a loopback device read-only and mount it read-only
this means nothing, ext4 will still write through to the backing storage
file.

Yes, I lost 6h debugging time on this.

Apparently, we have to specify "norecovery" when mounting such file
systems, to force them into truly read-only mode. Let's do so.