loop-util: LOOP_CONFIGURE ignores lo_sizelimit

loop-util: LOOP_CONFIGURE ignores lo_sizelimit

It appears LOOP_CONFIGURE in 5.8 is even more broken than initially
thought: it doesn't properly propgate lo_sizelimit to the block device
layer. :-(

Let's hence check the block device size immediately after issuing
LOOP_CONFIGURE, and if it doesn't match what we just set let's fallback
to the old ioctls.

This means LOOP_CONFIGURE currently works correctly only for the most
simply case: no partition table logic and no size limit. Sad!

(Kernel people should really be told about the concepts of tests and
even CI, one day!)