fib6_del_route has redundant code

[Gui Jianfeng <guijianfeng@cn.fujitsu.com>]

Hi all,
I think the following code in fib6_del_route in the latest kernel is useless.
1125         if (fn->leaf == NULL && fn->fn_flags&RTN_TL_ROOT)
1126                 fn->leaf = &ip6_null_entry;

ip6_null_entry will never be unlinked from fn->leaf now, that is, fn->leaf == NULL will never meet.

In previous kernel, When adding a default route, ip6_null_entry will be unlinked from fn->leaf.
So, when deleting a default route, it need to check whether the deleted route is the last one,
if so, ip6_null_entry will link to fn->leaf again.

I am not sure if there is another place unlinks ip6_null_entry from fn->leaf.

Regards,
Gui Jiafeng