Bluetooth: Reinitialize the list after deletion for session user list

If the user->list is deleted with list_del(), it doesn't initialize the
entry which can cause the issue with list_empty(). According to the
comment from the list.h, list_empty() returns false even if the list is
empty and put the entry in an undefined state.

/**
 * list_del - deletes entry from list.
 * @entry: the element to delete from the list.
 * Note: list_empty() on entry does not return true after this, the entry is
 * in an undefined state.
 */

Because of this behavior, list_empty() returns false even if list is empty
when the device is reconnected.

So, user->list needs to be re-initialized after list_del(). list.h already
have a macro list_del_init() which deletes the entry and initailze it again.
Signed-off-by: Tedd Ho-Jeong An <tedd.an@intel.com>
Tested-by: Jörg Otte <jrg.otte@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/l2cap_core.c

@@ -1634,7 +1634,7 @@ void l2cap_unregister_user(struct l2cap_conn *conn, struct l2cap_user *user)
 	if (list_empty(&user->list))
 		goto out_unlock;
 
-	list_del(&user->list);
+	list_del_init(&user->list);
 	user->remove(conn, user);
 
 out_unlock:
@@ -1648,7 +1648,7 @@ static void l2cap_unregister_all_users(struct l2cap_conn *conn)
 
 	while (!list_empty(&conn->users)) {
 		user = list_first_entry(&conn->users, struct l2cap_user, list);
-		list_del(&user->list);
+		list_del_init(&user->list);
 		user->remove(conn, user);
 	}
 }