ipv6: fix possible infinite loop in rt6_fill_node()

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

ipv6: fix possible infinite loop in rt6_fill_node()

Sashiko reported this issue [1]. Apply the same fix as

commit f8d8ce1b515a ("ipv6: fix possible infinite loop in fib6_info_uses_dev()").

Writers holding tb6_lock can list_del_rcu(&rt->fib6_siblings)

without waiting for RCU readers; rt->fib6_siblings.next then still

points into the old ring and this softirq-side walker never reaches

&rt->fib6_siblings, causing a CPU stall. fib6_del_route() always

WRITE_ONCE()s rt->fib6_nsiblings to 0 before list_del_rcu(), so an

inside-loop check is a reliable detach signal.

[1] https://sashiko.dev/#/patchset/20260526020227.4857-1-jiayuan.chen%40linux.dev