ipv6: fix possible infinite loop in fib6_info_uses_dev()

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

ipv6: fix possible infinite loop in fib6_info_uses_dev()

fib6_info_uses_dev() seems to rely on RCU without an explicit

protection.

Like the prior fix in rt6_nlmsg_size(),

we need to make sure fib6_del_route() or fib6_add_rt2node()

have not removed the anchor from the list, or we risk an infinite loop.