jump_label: disable preemption around __module_text_address().

Steven reported a warning caused by not holding module_mutex or
rcu_read_lock_sched: his backtrace was corrupted but a quick audit
found this possible cause.  It's wrong anyway...
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

kernel/jump_label.c

@@ -284,11 +284,14 @@ static int __jump_label_mod_text_reserved(void *start, void *end)
 {
 	struct module *mod;
 
+	preempt_disable();
 	mod = __module_text_address((unsigned long)start);
+	WARN_ON_ONCE(__module_text_address((unsigned long)end) != mod);
+	preempt_enable();
+
 	if (!mod)
 		return 0;
 
-	WARN_ON_ONCE(__module_text_address((unsigned long)end) != mod);
 
 	return __jump_label_text_reserved(mod->jump_entries,
 				mod->jump_entries + mod->num_jump_entries,