nixos/swap: remove fallocate and use dd as the main swap creation method

https://github.com/NixOS/nixpkgs/issues/144572
https://github.com/NixOS/nixpkgs/issues/91986

nixos/swap: remove expansion if else because the file is already
recreated if file size is not the same as the configured size

nixos/swap: remove old fixme

nixos/modules/config/swap.nix

@@ -203,7 +203,6 @@ in
     ];
 
     # Create missing swapfiles.
-    # FIXME: support changing the size of existing swapfiles.
     systemd.services =
       let
 
@@ -223,11 +222,7 @@ in
                 ${optionalString (sw.size != null) ''
                   currentSize=$(( $(stat -c "%s" "${sw.device}" 2>/dev/null || echo 0) / 1024 / 1024 ))
                   if [ "${toString sw.size}" != "$currentSize" ]; then
-                    fallocate -l ${toString sw.size}M "${sw.device}" ||
-                      dd if=/dev/zero of="${sw.device}" bs=1M count=${toString sw.size}
-                    if [ "${toString sw.size}" -lt "$currentSize" ]; then
-                      truncate --size "${toString sw.size}M" "${sw.device}"
-                    fi
+                    dd if=/dev/zero of="${sw.device}" bs=1M count=${toString sw.size}
                     chmod 0600 ${sw.device}
                     ${optionalString (!sw.randomEncryption.enable) "mkswap ${sw.realDevice}"}
                   fi