x86: Add stack top margin for stack overflow checking

It seems that a margin for stack overflow checking is added to
top of a kernel stack but is not added to IRQ and exception
stacks in stack_overflow_check(). Therefore, the overflows of
IRQ and exception stacks are always detected only after they
actually occurred and data corruption might occur due to them.

This patch adds the margin to top of IRQ and exception stacks
as well as a kernel stack to enhance reliability.

Signed-off-by: Mitsuo Hayasaka <mitsuo.hayasaka.hu@hitachi.com>
Cc: yrl.pp-manager.tt@hitachi.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20111207082910.9847.3359.stgit@ltc219.sdl.hitachi.co.jp


[ removed the #undef - we typically don't do that for uncommon names ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>