dissect: tighten block device checks a bit

dissect: tighten block device checks a bit

This extends on #8609, and makes two changes:

1. We'll now explicitly check that the child devices of a block device
   we are interested in (i.e. the partitions) are block devices themselves.
   On newer kernels the mmc rpmb stuff is actually exposed as char rather
   than block device as before, and they probably should have been that in
   the first place. By adding this check we'll hence filter out these weird
   devices through a second rule too, that hopefully makes things a bit
   more future-proof, should more devices like this be added eventually,
   or other subsystems do a similar thing.

2. When counting partitions we'll now also check the devnum of the
   device being non-null, which we already do when matching up the devices
   in the second iteration. This should make things more robust, and
   prevent other kinds of miscounting, which after all was the main
   issue #8609 fixed.