compat: redefine version constants for sublevel>=256

With the 4.4.256 and 4.9.256 kernels, the previous calculation for
integer comparison overflowed. This commit redefines the broken
constants to have more space for the sublevel.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Author: zx2c4

src/compat/Kbuild.include

@@ -6,6 +6,7 @@ kbuild-dir := $(if $(filter /%,$(src)),$(src),$(srctree)/$(src))
 
 ccflags-y += -include $(kbuild-dir)/compat/compat.h
 asflags-y += -include $(kbuild-dir)/compat/compat-asm.h
+LINUXINCLUDE := -DCOMPAT_VERSION=$(VERSION) -DCOMPAT_PATCHLEVEL=$(PATCHLEVEL) -DCOMPAT_SUBLEVEL=$(SUBLEVEL) -I$(kbuild-dir)/compat/version $(LINUXINCLUDE)
 
 ifeq ($(wildcard $(srctree)/include/linux/ptr_ring.h),)
 ccflags-y += -I$(kbuild-dir)/compat/ptr_ring/include

src/compat/version/linux/version.h

@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2015-2021 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+#include_next <linux/version.h>
+#undef KERNEL_VERSION
+#define KERNEL_VERSION(a, b, c) (((a) << 24) + ((b) << 16) + (c))
+#undef LINUX_VERSION_CODE
+#define LINUX_VERSION_CODE KERNEL_VERSION(COMPAT_VERSION, COMPAT_PATCHLEVEL, COMPAT_SUBLEVEL)