[PATCH] x86_64: Fix backtracing for interrupt stacks

Re-add backlink for old style unwinder to stack switching.  Add proper
stack frame and CFI annotations to call_softirq

This prevents a oops when backtracing with fallback through the
interrupt stack top.

Suggested by Jan Beulich and Herbert Xu wanted it in 2.6.18.

Cc: jbeulich@novell.com
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>