ring-buffer: Make rb_reserve_next_event() always inlined

The function rb_reserved_next_event() is called by two functions:
ring_buffer_lock_reserve() and ring_buffer_write(). This is in a very hot
path of the tracing code, and it is best that they are not functions. The
two callers are basically wrapers for rb_reserver_next_event(). Removing the
function calls can save execution time in the hotpath of tracing.

Link: http://lkml.kernel.org/r/20161121183700.GW26852@two.firstfloor.org

Reported-by: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/ring_buffer.c

@@ -2733,7 +2733,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
 	return event;
 }
 
-static struct ring_buffer_event *
+static __always_inline struct ring_buffer_event *
 rb_reserve_next_event(struct ring_buffer *buffer,
 		      struct ring_buffer_per_cpu *cpu_buffer,
 		      unsigned long length)