sysext: Use correct image name for extension release checks

sysext: Use correct image name for extension release checks

For the extension release check the image name is needed and was derived
from the backing file of the loop device. However, this can have a
different name when symlinks were resolved. The surprising behavior was
that it worked when the target name started with the extension name and
_ because that's what's supported to chop off version suffixes. However,
we should not have such strict requirements for the target name and also
allow - as version separator and entirely different names/prefixes, the
same way as we also do for directories instead of raw images.

Do not use the image name derived from the backing file of the loop
device but directly the extension name we have at hand.