ipv6: prevent infinite loop in rt6_nlmsg_size()

In the Linux kernel, the following vulnerability has been resolved:

ipv6: prevent infinite loop in rt6_nlmsg_size()

While testing prior patch, I was able to trigger an infinite loop in rt6_nlmsg_size() in the following place:

list_for_each_entry_rcu(sibling, &f6i->fib6_siblings, fib6_siblings) { rt6_nh_nlmsg_size(sibling->fib6_nh, &nexthop_len); }

This is because fib6_del_route() and fib6_add_rt2node() uses list_del_rcu(), which can confuse rcu readers, because they might no longer see the head of the list.

Restart the loop if f6i->fib6_nsiblings is zero.