[PATCH] x86_64: Rewrite exception stack backtracing

Exceptions and hardware interrupts can, to a certain degree, nest, so when
attempting to follow the sequence of stacks used in order to dump their
contents this has to be accounted for.  Also, IST stacks have their tops
stored in the TSS, so there's no need to add the stack size to get to their
ends.

Minor changes from AK.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>