x86: Don't call dump_stack() from arch_trigger_all_cpu_backtrace_handler()

show_regs() already prints two(!) stack traces, no need for a third one.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
LKML-Reference: <4D5D512902000078000326EE@vpn.id2.novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>