spl: fix build with linux 5.1

Upstream issue: https://github.com/zfsonlinux/zfs/issues/8697
Upstream fix (zfs repo): 782dfae3218b5f2029ce78722b999cb04e8ef001

This can't be applied cleanly as a patch, since spl has been moved
into the zfs repo since 0.7.13.

pkgs/os-specific/linux/spl/default.nix

@@ -21,6 +21,11 @@ stdenv.mkDerivation rec {
 
   patches = [ ./install_prefix.patch ];
 
+  # Backported fix for 0.7.13 to build with 5.1, please remove when updating to 0.7.14
+  postPatch = optionalString (versionAtLeast kernel.version "5.1") ''
+    sed -i 's/get_ds()/KERNEL_DS/g' module/spl/spl-vnode.c
+  '';
+
   nativeBuildInputs = [ autoreconfHook ] ++ kernel.moduleBuildDependencies;
 
   hardeningDisable = [ "fortify" "stackprotector" "pic" ];