dissect: when we invoke dissection on a loop device with partscan help the user

dissect: when we invoke dissection on a loop device with partscan help the user

This adds some simply detection logic for cases where dissection is
invoked on an externally created loop device, and partitions have been
detected on it, but partition scanning so far was off. If this is
detected we now print a brief message indicating what the issue is,
instead of failing with a useless EINVAL message the kernel passed to
us.