tracepoint: Fix out of sync data passing by static caller

Naresh reported a bug that appears to be a side effect of the static
calls. It happens when going from more than one tracepoint callback to
a single one, and removing the first callback on the list. The list of
tracepoint callbacks holds data and a function to call with the
parameters of that tracepoint and a handler to the associated data.

 old_list:
	0: func = foo; data = NULL;
	1: func = bar; data = &bar_struct;

 new_list:
	0: func = bar; data = &bar_struct;

	CPU 0				CPU 1
	-----				-----
   tp_funcs = old_list;
   tp_static_caller = tp_interator

   __DO_TRACE()

    data = tp_funcs[0].data = NULL;

				   tp_funcs = new_list;
				   tracepoint_update_call()
				      tp_static_caller = tp_funcs[0] = bar;
    tp_static_caller(data)
       bar(data)
         x = data->item = NULL->item

       BOOM!

To solve this, add a tracepoint_synchronize_unregister() between
changing tp_funcs and updating the static tracepoint, that does both a
synchro...