nixos/zram: add zramSwap.memoryMax option

This allows capping the total amount of memory that will be used for
zram-swap, in addition to the percentage-based calculation, which is
useful when blanket-applying a configuration to many machines.

This is based off the strategy used by Fedora for their rollout of
zram-swap-by-default in Fedora 33
(https://fedoraproject.org/wiki/Changes/SwapOnZRAM), which caps the
maximum amount of memory used for zram at 4GiB.

In future it might be good to port this to the systemd zram-generator,
instead of using this separate infrastructure.

nixos/modules/config/zram.nix

@@ -80,6 +80,15 @@ in
         '';
       };
 
+      memoryMax = mkOption {
+        default = null;
+        type = with types; nullOr int;
+        description = ''
+          Maximum total amount of memory (in bytes) that can be used by the zram
+          swap devices.
+        '';
+      };
+
       priority = mkOption {
         default = 5;
         type = types.int;
@@ -146,7 +155,12 @@ in
 
               # Calculate memory to use for zram
               mem=$(${pkgs.gawk}/bin/awk '/MemTotal: / {
-                  print int($2*${toString cfg.memoryPercent}/100.0/${toString devicesCount}*1024)
+                  value=int($2*${toString cfg.memoryPercent}/100.0/${toString devicesCount}*1024);
+                    ${lib.optionalString (cfg.memoryMax != null) ''
+                      memory_max=int(${toString cfg.memoryMax}/${toString devicesCount});
+                      if (value > memory_max) { value = memory_max }
+                    ''}
+                  print value
               }' /proc/meminfo)
 
               ${pkgs.util-linux}/sbin/zramctl --size $mem --algorithm ${cfg.algorithm} /dev/${dev}