kernel config: add support for CONFIG_SQUASHFS_ZSTD (#52967)

Also, allow override `make-squashfs.nix` compression parameters.

nixos/lib/make-squashfs.nix

@@ -3,6 +3,9 @@
 , # The root directory of the squashfs filesystem is filled with the
   # closures of the Nix store paths listed here.
   storeContents ? []
+, # Compression parameters.
+  # For zstd compression you can use "zstd -Xcompression-level 6".
+  comp ? "xz -Xdict-size 100%"
 }:
 
 stdenv.mkDerivation {
@@ -20,6 +23,6 @@ stdenv.mkDerivation {
 
       # Generate the squashfs image.
       mksquashfs nix-path-registration $(cat $closureInfo/store-paths) $out \
-        -keep-as-directory -all-root -b 1048576 -comp xz -Xdict-size 100%
+        -keep-as-directory -all-root -b 1048576 -comp ${comp}
     '';
 }

pkgs/os-specific/linux/kernel/common-config.nix

@@ -320,6 +320,7 @@ let
       SQUASHFS_LZO                 = yes;
       SQUASHFS_XZ                  = yes;
       SQUASHFS_LZ4                 = yes;
+      SQUASHFS_ZSTD                = whenAtLeast "4.14" yes;
 
       # Native Language Support modules, needed by some filesystems
       NLS              = yes;