ipv6: anycast: Don't use rtnl_dereference().

ipv6: anycast: Don't use rtnl_dereference().

inet6_dev->ac_list is protected by inet6_dev->lock, so rtnl_dereference()
is a bit rough annotation.

As done in mcast.c, we can use ac_dereference() that checks if
inet6_dev->lock is held.

Let's replace rtnl_dereference() with a new helper ac_dereference().

Note that now addrconf_join_solict() / addrconf_leave_solict() in
__ipv6_dev_ac_inc() / __ipv6_dev_ac_dec() does not need RTNL, so we
can remove ASSERT_RTNL() there.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250702230210.3115355-12-kuni1840@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>