tracing/rb: Convert to hotplug state machine

Install the callbacks via the state machine. The notifier in struct
ring_buffer is replaced by the multi instance interface.  Upon
__ring_buffer_alloc() invocation, cpuhp_state_add_instance() will invoke
the trace_rb_cpu_prepare() on each CPU.

This callback may now fail. This means __ring_buffer_alloc() will fail and
cleanup (like previously) and during a CPU up event this failure will not
allow the CPU to come up.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20161126231350.10321-7-bigeasy@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/ring_buffer.h

@@ -198,4 +198,10 @@ enum ring_buffer_flags {
 	RB_FL_OVERWRITE		= 1 << 0,
 };
 
+#ifdef CONFIG_RING_BUFFER
+int trace_rb_cpu_prepare(unsigned int cpu, struct hlist_node *node);
+#else
+#define trace_rb_cpu_prepare	NULL
+#endif
+
 #endif /* _LINUX_RING_BUFFER_H */