nixos/swap: use btrfs mkswapfile if possible

          sw:

          let

            realDevice' = utils.escapeSystemdPath sw.realDevice;

            btrfsInSystem = config.boot.supportedFilesystems.btrfs or false;

          in

          lib.nameValuePair "mkswap-${sw.deviceName}" {

            description = "Initialisation of swap device ${sw.device}";

              pkgs.util-linux

              pkgs.e2fsprogs

            ]

            ++ lib.optional btrfsInSystem pkgs.btrfs-progs

            ++ lib.optional sw.randomEncryption.enable pkgs.cryptsetup;

            environment.DEVICE = sw.device;

              ${lib.optionalString (sw.size != null) ''

                currentSize=$(( $(stat -c "%s" "$DEVICE" 2>/dev/null || echo 0) / 1024 / 1024 ))

                if [[ ! -b "$DEVICE" && "${toString sw.size}" != "$currentSize" ]]; then

                  # Disable CoW for CoW based filesystems like BTRFS.

                  if [[ $(stat -f -c %T $(dirname "$DEVICE")) == "btrfs" ]]; then

                    # Use btrfs mkswapfile to speed up the creation of swapfile.

                    rm -f "$DEVICE"

                    btrfs filesystem mkswapfile --size "${toString sw.size}M" --uuid clear "$DEVICE"

                  else

                    # Disable CoW for CoW based filesystems.

                    truncate --size 0 "$DEVICE"

                    chattr +C "$DEVICE" 2>/dev/null || true

                    dd if=/dev/zero of="$DEVICE" bs=1M count=${toString sw.size} status=progress

                    ${lib.optionalString (!sw.randomEncryption.enable) "mkswap ${sw.realDevice}"}

                  fi

                fi

              ''}

              ${lib.optionalString sw.randomEncryption.enable ''

                cryptsetup plainOpen -c ${sw.randomEncryption.cipher} -d ${sw.randomEncryption.source} \