make sure all swap units are ordered before the swap target

make sure all swap units are ordered before the swap target

When shutting down the system, the swap devices can be disabled long
time before the swap target is stopped. They're actually the first
units systemd turns off on my system.

This is incorrect and due to swap devices having multiple associated
swap unit files. The main one is usually created by the fstab
generator and is used to start the swap device.

Once done, systemd creates some 'alias' units for the same swap
device, one for each swap dev link. But those units are missing an
ordering dependencies which was created by the fstab generator for the
main swap unit.

Therefore during shutdown those 'alias' units can be stopped at
anytime before unmount.target target.

This patch makes sure that all swap units are stopped after the
swap.target target.