nixos/swap: support for resizing swapfile

Currently NixOS creates the swapfile (with the specified size) only if
it doesn't already exist. Changing the swapfile size afterwards will not
have any effect.

This commit changes that so the swapfile will be recreated whenever
swapDevices.*.size is changed (or more precisely, whenever the actual
file size differs from the configured one), allowing both growing and
shrinking the swapfile.

The service unit has "restartIfChanged = false", so we don't have to
worry about the swapfile being in use at the time this code is run (you
have to reboot for swapfile changes).

fallocate doesn't shrink files, use truncate for that. truncate can also
be used to grow files, but it creates "holes" in the file which doesn't
work with swapfiles.