ipv6: anycast: Unify two error paths in ipv6_sock_ac_join().

ipv6: anycast: Unify two error paths in ipv6_sock_ac_join().

The next patch will replace __dev_get_by_index() and __dev_get_by_flags()
to RCU + refcount version.

Then, we will need to call dev_put() in some error paths.

Let's unify two error paths to make the next patch cleaner.

Note that we add READ_ONCE() for net->ipv6.devconf_all->forwarding
and idev->conf.forwarding as we will drop RTNL that protects them.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250702230210.3115355-14-kuni1840@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>