powerpc/ftrace: Fix the comments for ftrace_modify_code

There is no need to worry about module and __init text disappearing
case, because that ftrace has a module notifier that is called when a
module is being unloaded and before the text goes away and this code
grabs the ftrace_lock mutex and removes the module functions from the
ftrace list, such that it will no longer do any modifications to that
module's text, the update to make functions be traced or not is done
under the ftrace_lock mutex as well. And by now, __init section codes
should not been modified by ftrace, because it is black listed in
recordmcount.c and ignored by ftrace.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Li Bin <huawei.libin@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>