ftrace: Optimize the function tracer list loop

There is lots of places that perform:

       op = rcu_dereference_raw(ftrace_control_list);
       while (op != &ftrace_list_end) {

Add a helper macro to do this, and also optimize for a single
entity. That is, gcc will optimize a loop for either no iterations
or more than one iteration. But usually only a single callback
is registered to the function tracer, thus the optimized case
should be a single pass. to do this we now do:

	op = rcu_dereference_raw(list);
	do {
		[...]
	} while (likely(op = rcu_dereference_raw((op)->next)) &&
	       unlikely((op) != &ftrace_list_end));

An op is always registered (ftrace_list_end when no callbacks is
registered), thus when a single callback is registered, the link
list looks like:

 top => callback => ftrace_list_end => NULL.

The likely(op = op->next) still must be performed due to the race
of removing the callback, where the first op assignment could
equal ftrace_list_end. In that case, the op->next would be NULL.
But this is unlikely (only happens in a race condition when
removing the callback).

But it is very likely that the next op would be ftrace_list_end,
unless more than one callback has been registered. This tells
gcc what the most common case is and makes the fast path with
the least amount of branches.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>