ipv6: annotate data-races around rt->fib6_nsiblings

rt->fib6_nsiblings can be read locklessly, add corresponding
READ_ONCE() and WRITE_ONCE() annotations.

Fixes: 66f5d6ce ("ipv6: replace rwlock with rcu and spinlock in fib6_table")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250725140725.3626540-5-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>