zram: revert "change default algorithm to zstd" (#56856)

19.03 default kernel is still 4.14, which doesn't support zstd. So,
zramSwap in current fasion fails on default kernel.

nixos/doc/manual/release-notes/rl-1903.xml

@@ -577,7 +577,7 @@
        but is still possible by setting <literal>zramSwap.swapDevices</literal> explicitly.
      </para>
      <para>
-      Default algorithm for ZRAM swap was changed to <literal>zstd</literal>.
+      ZRAM algorithm can be changed now.
      </para>
      <para>
       Changes to ZRAM algorithm are applied during <literal>nixos-rebuild switch</literal>,

nixos/modules/config/zram.nix

@@ -91,13 +91,13 @@ in
       };
 
       algorithm = mkOption {
-        default = "zstd";
-        example = "lzo";
+        default = "lzo";
+        example = "lz4";
         type = with types; either (enum [ "lzo" "lz4" "zstd" ]) str;
         description = ''
           Compression algorithm. <literal>lzo</literal> has good compression,
           but is slow. <literal>lz4</literal> has bad compression, but is fast.
-          <literal>zstd</literal> is both good compression and fast.
+          <literal>zstd</literal> is both good compression and fast, but requires newer kernel.
           You can check what other algorithms are supported by your zram device with
           <programlisting>cat /sys/class/block/zram*/comp_algorithm</programlisting>
         '';