linux_5_0: restore __kernel_fpu_{begin,restore}

In 5.0er these function were removed from the public interface also zfs needs
them for AVX/AES-NI support. Without this patch for example throughput on a
encrypted zfs dataset drops to 200 MB/s from 1.2 GB/s. These functions were
removed as their was no user within the linux kernel tree itself.

pkgs/os-specific/linux/kernel/patches.nix

@@ -57,4 +57,9 @@ rec {
       sha256 = "1l8xq02rd7vakxg52xm9g4zng0ald866rpgm8kjlh88mwwyjkrwv";
     };
   };
+
+  export_kernel_fpu_functions = rec {
+    name = "export_kernel_fpu_functions";
+    patch = ./export_kernel_fpu_functions.patch;
+  };
 }