ppc: Implement save_stack_trace_regs()

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 'r exit_mmap' >> kprobe_events
 # echo 1 > events/kprobes/enable

Gives the following splat:

save_stack_trace_regs() not implemented yet.
------------[ cut here ]------------
WARNING: at kernel/stacktrace.c:74
Modules linked in:
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.3.0-test torvalds#89
task: c000000000a98660 ti: c00000003efe8000 task.ti: c000000000b98000
NIP: c0000000000fa72c LR: c0000000000fa728 CTR: c0000000004a8b80
REGS: c00000003efeaf40 TRAP: 0700   Not tainted  (4.3.0-test)
MSR: 9000000000021032 <SF,HV,ME,IR,DR,RI>  CR: 28002042  XER: 20000000
SOFTE: 0
GPR00: c0000000000fa728 c00000003efeb1c0 c000000000b9e400 000000000000002c
GPR04: 0000000000000000 000000000000013b c000000000b5e790 6e6f7420696d706c
GPR08: 656d656e00000000 c000000000aad148 c000000000ba3878 00000000000001ef
GPR12: 0000000022002044 c00000000ffff000 0000000000000100 0000000000000000
GPR16: c00000003800f01c c00000003800f02c 000000010000a849 c000000038008100
GPR20: c00000003800f010 c00000003efe8000 0000000000000000 0000000000000000
GPR24: 0000000000000100 c0000000389ad120 c0000000389ad1b0 0000000000000000
GPR28: 0000000000000100 c00000003efeb920 c000000000a1d88c c000000000a93529
NIP [c0000000000fa72c] .save_stack_trace_regs+0x3c/0x70
LR [c0000000000fa728] .save_stack_trace_regs+0x38/0x70
Call Trace:
[c00000003efeb1c0] [c0000000000fa728] .save_stack_trace_regs+0x38/0x70 (unreliable)
[c00000003efeb240] [c000000000145b2c] .__ftrace_trace_stack+0x15c/0x210
[c00000003efeb310] [c000000000145cc4] .ftrace_trace_stack_regs+0x24/0x40
[c00000003efeb380] [c0000000001467c4] .trace_buffer_unlock_commit_regs+0x44/0x70
[c00000003efeb420] [c000000000165b38] .kprobe_trace_func+0x278/0x400
[c00000003efeb530] [c000000000165d38] .kprobe_dispatcher+0x78/0xa0
[c00000003efeb5c0] [c0000000007b0ca0] .kprobe_exceptions_notify+0x2e0/0x520
[c00000003efeb670] [c0000000000b4934] .notifier_call_chain+0x94/0xf0
[c00000003efeb710] [c0000000000b4a2c] .atomic_notifier_call_chain+0x3c/0x50
[c00000003efeb7a0] [c0000000000b4c48] .notify_die+0x38/0x50
[c00000003efeb830] [c0000000007aff90] .program_check_exception+0x1a0/0x260
[c00000003efeb8b0] [c00000000000621c] program_check_common+0x11c/0x180

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[PT_R1], current, 0);
+}
+EXPORT_SYMBOL_GPL(save_stack_trace_regs);