[PATCH] x86_64: fix page fault from show_trace()

The introduction of call_softirq switching to the interrupt stack several
releases earlier resulted in a problem with the code in show_trace, which
assumes that it can pick the previous stack pointer from the end of the
interrupt stack.

Cc: Andi Kleen <ak@muc.de>
Cc: Arjan van de Ven <arjanv@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>