Module: check to see if we have a built in module with the same name

When trying to load a module with the same name as a built-in one, a
scary kobject backtrace comes up.  Prevent that from checking for this
condition and warning the user as to what exactly is going on.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

kernel/module.c

@@ -1230,13 +1230,23 @@ void module_remove_modinfo_attrs(struct module *mod)
 int mod_sysfs_init(struct module *mod)
 {
 	int err;
+	struct kobject *kobj;
 
 	if (!module_sysfs_initialized) {
 		printk(KERN_ERR "%s: module sysfs not initialized\n",
 		       mod->name);
 		err = -EINVAL;
 		goto out;
 	}
+
+	kobj = kset_find_obj(module_kset, mod->name);
+	if (kobj) {
+		printk(KERN_ERR "%s: module is already loaded\n", mod->name);
+		kobject_put(kobj);
+		err = -EINVAL;
+		goto out;
+	}
+
 	mod->mkobj.mod = mod;
 
 	memset(&mod->mkobj.kobj, 0, sizeof(mod->mkobj.kobj));