powerpc: Implement save_stack_trace_regs() to enable kprobe stack tracing

It has come to my attention that kprobe event stack tracing does not
work on powerpc. You can see with the following:

  # cd /sys/kernel/debug/tracing
  # echo stacktrace > trace_options
  # echo 'p kfree' > kprobe_events
  # echo 1 > events/kprobes/enable

Will print the following warning:
  save_stack_trace_regs() not implemented yet.

Although save_stack_trace() (which normal event stack traces use) is
implemented, save_stack_trace_regs() which kprobe events use is not.
This is a cheap attempt to implement that function.

Note, This may have issues if a task tries to get a stack trace from
another task with its regs, because it just passes in "current" to
save_context_stack(). But this does solve the issue with stack tracing
kprobe events.
Reported-by: Chunyu Hu <chuhu@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/stacktrace.c

@@ -61,3 +61,10 @@ void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
 	save_context_stack(trace, tsk->thread.ksp, tsk, 0);
 }
 EXPORT_SYMBOL_GPL(save_stack_trace_tsk);
+
+void
+save_stack_trace_regs(struct pt_regs *regs, struct stack_trace *trace)
+{
+	save_context_stack(trace, regs->gpr[1], current, 0);
+}
+EXPORT_SYMBOL_GPL(save_stack_trace_regs);