kernel.h: drop STACK_MAGIC macro

kernel.h: drop STACK_MAGIC macro

Patch series "Unload linux/kernel.h", v5.

kernel.h hosts declarations that can be placed better.  This series
decouples kernel.h with some explicit and implicit dependencies; also,
moves tracing functionality to a new independent header.

This patch (of 6):

The macro was introduced in 1994, v1.0.4, for stacks protection.  Since
that, people found better ways to protect stacks, and now the macro is
only used by i915 selftests.  Move it to a local header and drop from the
kernel.h.

Link: https://lkml.kernel.org/r/20260116042510.241009-1-ynorov@nvidia.com
Link: https://lkml.kernel.org/r/20260116042510.241009-2-ynorov@nvidia.com
Signed-off-by: Yury Norov <ynorov@nvidia.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Reviewed-by: Aaron Tomlin <atomlin@atomlin.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Petr Pavlu <petr.pavlu@suse.com>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>