swap: always add in extras when we load a swap unit

swap: always add in extras when we load a swap unit

Much like for the mount units we need fields such as the slice
initialized by the time we activate the swap, hence when the kernel
let's us know about a new swap that appeared we need to initialize the
slice in any Swap object we allocated for that right-away, even if we
can't read the real unit file for the swap device.