fprobe: Ensure running fprobe_exit_handler() finished before calling rethook_free()

Ensure running fprobe_exit_handler() has finished before
calling rethook_free() in the unregister_fprobe() so that caller can free
the fprobe right after unregister_fprobe().

unregister_fprobe() ensured that all running fprobe_entry/exit_handler()
have finished by calling unregister_ftrace_function() which synchronizes
RCU. But commit 5f810187 ("fprobe: Release rethook after the ftrace_ops
is unregistered") changed to call rethook_free() after
unregister_ftrace_function(). So call rethook_stop() to make rethook
disabled before unregister_ftrace_function() and ensure it again.

Here is the possible code flow that can call the exit handler after
unregister_fprobe().

------
 CPU1                              CPU2
 call unregister_fprobe(fp)
 ...
                                   __fprobe_handler()
                                   rethook_hook() on probed function
 unregister_ftrace_function()
                                   return from probed function
                                   rethook hooks
                                   find rh->handler == fprobe_exit_handler
                                   call fprobe_exit_handler()
 rethook_free():
   set rh->handler = NULL;
 return from unreigster_fprobe;
                                   call fp->exit_handler() <- (*)
------

(*) At this point, the exit handler is called after returning from
unregister_fprobe().

This fixes it as following;
------
 CPU1                              CPU2
 call unregister_fprobe()
 ...
 rethook_stop():
   set rh->handler = NULL;
                                   __fprobe_handler()
                                   rethook_hook() on probed function
 unregister_ftrace_function()
                                   return from probed function
                                   rethook hooks
                                   find rh->handler == NULL
                                   return from rethook
 rethook_free()
 return from unreigster_fprobe;
------

Link: https://lore.kernel.org/all/168873859949.156157.13039240432299335849.stgit@devnote2/



Fixes: 5f810187 ("fprobe: Release rethook after the ftrace_ops is unregistered")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>