Fix swapfile "insecure permissions"

This isn't really necessary as /swap is mode 0700, but a warning is
logged in the journal[1], so let's set /swap/swapfile to mode 0600 and
get rid of the warning.

[1] swapon[189]: swapon: /swap/swapfile: insecure permissions 0644, 0600 suggested.
5 jobs for swap-perms in 24 minutes and 18 seconds (queued for 2 seconds)