ftrace: x86 use copy to and from user functions

The modification of code is performed either by kstop_machine, before
SMP starts, or on module code before the module is executed. There is
no reason to do the modifications from assembly. The copy to and from
user functions are sufficient and produces cleaner and easier to read
code.

Thanks to Benjamin Herrenschmidt for suggesting the idea.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>