I'm sorry for having to resend this, but I mistakenly sent an older version of the patch. The correct one is attached
now, fixing a warning and a build problem when CONFIG_DOUBLEFAULT is not defined.

From: Jan Beulich <jbeulich@novell.com>

Make the double fault handler use CPU-specific stacks. Add some
abstraction to simplify future change of other exception handlers to go
through task gates. Change the pointer validity checks in the double
fault handler to account for the fact that both GDT and TSS aren't in
static kernel space anymore. Add a new notification of the event
through the die notifier chain, also providing some environmental
adjustments so that various infrastructural things work independent of
the fact that the fault and the callbacks are running on other then the
normal kernel stack.

Signed-Off-By: Jan Beulich <jbeulich@novell.com>