linux_4_19: restore __kernel_fpu_{begin,restore}

Linux commit 12209993e98c5fa1855c467f22a24e3d5b8be205 was backported to
4.19, so add the reverting patch to restore zfs performance.

pkgs/top-level/all-packages.nix

@@ -15323,6 +15323,7 @@ in
     kernelPatches =
       [ kernelPatches.bridge_stp_helper
         kernelPatches.modinst_arg_list_too_long
+        kernelPatches.export_kernel_fpu_functions
       ];
   };