udev: reset loopback block device ownership and mode on detach

udev: reset loopback block device ownership and mode on detach

Loopback block devices are agressively reused, without being removed in
between. This means various inode attributes on their device nodes will
– so far – remain in effect between uses of the devices. Since there are
applications which change access mode/ownership of such devices after
attaching files to them, let's undo this again when we detect them to be
unused again.

Fixes: #37745