udisks2: Fix some memory leaks

https://bugzilla.redhat.com/show_bug.cgi?id=790062



This was done using the following rather simple script

 #!/bin/bash
 while true; do
     losetup -f /home/davidz/Downloads/Fedora-16-x86_64-Live-Desktop.iso
     losetup -D
 done

and then running it as root.

Signed-off-by: David Zeuthen <davidz@redhat.com>