ftrace: Add a ftrace_free_mem() function for modules to use

In order to be able to trace module init functions, the module code needs to
tell ftrace what is being freed when the init sections are freed. Use the
code that the main init calls to tell ftrace to free the main init sections.
This requires passing in a start and end address to free.

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