zfs: pass kernel build flags correctly

ZFS's autoconf scripts recognize KERNEL_CC, KERNEL_CROSS_COMPILE,
KERNEL_ARCH, etc.  This avoids the need to separately give them as
Make flags, although I'm not sure what the original intention behind
giving the unprefixed flags to configure was — the commit adding that
doesn't say, except that it was to fix cross, which still works after
this change.

This fixes the ZFS kernel module build after the change to use
unwrapped compilers for building the kernel, which previously resulted
in CC etc. being clobbered for the ZFS build, even for non-kernel
parts like the "compiler works" Autoconf check.

pkgs/os-specific/linux/zfs/generic.nix

@@ -221,11 +221,9 @@ let
           "--with-linux=${kernel.dev}/lib/modules/${kernel.modDirVersion}/source"
           "--with-linux-obj=${kernel.dev}/lib/modules/${kernel.modDirVersion}/build"
         ]
-        ++ kernelModuleMakeFlags
+        ++ map (f: "KERNEL_${f}") kernelModuleMakeFlags
       );
 
-      makeFlags = optionals buildKernel kernelModuleMakeFlags;
-
       enableParallelBuilding = true;
 
       doInstallCheck = true;