kernel.h: include a note to discourage people from including it in headers

Include a note at the top to discourage people from including it in
headers.

Link: https://lkml.kernel.org/r/20211209150803.4473-1-andriy.shevchenko@linux.intel.comSigned-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/linux/kernel.h

 /* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * NOTE:
+ *
+ * This header has combined a lot of unrelated to each other stuff.
+ * The process of splitting its content is in progress while keeping
+ * backward compatibility. That's why it's highly recommended NOT to
+ * include this header inside another header file, especially under
+ * generic or architectural include/ directory.
+ */
 #ifndef _LINUX_KERNEL_H
 #define _LINUX_KERNEL_H