nixos/luksroot: Check whether the device already exists

The new reuse behaviour is cool and really useful but it breaks one of
my use cases. When using kexec, I have a script which will unlock the
disks in my initrd. However, do_open_passphrase will fail if the disk is
already unlocked.

nixos/modules/system/boot/luksroot.nix

@@ -118,6 +118,11 @@ let
     do_open_passphrase() {
         local passphrase
 
+        # Return if already unlocked
+        if ! cryptsetup status "${name}" > /dev/null; then
+          return
+        fi
+
         while true; do
             echo -n "Passphrase for ${device}: "
             passphrase=