ipv6: anycast: Don't hold RTNL for IPV6_JOIN_ANYCAST.

ipv6: anycast: Don't hold RTNL for IPV6_JOIN_ANYCAST.

inet6_sk(sk)->ipv6_ac_list is protected by lock_sock().

In ipv6_sock_ac_join(), only __dev_get_by_index(), __dev_get_by_flags(),
and __in6_dev_get() require RTNL.

__dev_get_by_flags() is only used by ipv6_sock_ac_join() and can be
converted to RCU version.

Let's replace RCU version helper and drop RTNL from IPV6_JOIN_ANYCAST.

setsockopt_needs_rtnl() will be removed in the next patch.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250702230210.3115355-15-kuni1840@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>