swap: always track the current real device node of all swap devices, even when not...

swap: always track the current real device node of all swap devices, even when not active

This way, we can avoid executing two /bin/swapon jobs to be dispatched
for the same swap device if it is configured for two different paths.

Previously we were just tracking the device nodes of active swap
devices, which would not allow us to recognize the identity of two swap
devices before they are active.

https://bugs.freedesktop.org/show_bug.cgi?id=69835