Impact: add SysRq-z support to dump trace buffers

Allows one to force an ftrace dump from sysrq
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>