zfs: fix patch inclusion

modules/zfs.nix

@@ -20,14 +20,19 @@ let
   arcMaxBytes = (totalRamBytes - totalHugepageBytes) * 60 / 100;
 in
 {
-  # remove inline dbuf_evict_one call so the dedicated eviction thread
+  # Patch ZFS via overlay so both userspace and kernel module are patched.
-  # handles cache pressure instead of stalling txg_sync on 6.14+.
+  # boot.zfs.package alone only affects userspace; the kernel module
+  # is resolved via boot.kernelPackages.zfs_2_4, which needs the overlay.
   # https://github.com/openzfs/zfs/issues/18426
-  boot.zfs.package = pkgs.zfs_2_4.overrideAttrs (old: {
+  nixpkgs.overlays = [
+    (final: prev: {
+      zfs_2_4 = prev.zfs_2_4.overrideAttrs (old: {
         patches = (old.patches or [ ]) ++ [
           ../patches/zfs/0001-remove-dbuf_evict_one-call.patch
         ];
       });
+    })
+  ];
   boot.initrd.kernelModules = [ "zfs" ];
 
   boot.kernelParams = [