ipv6: anycast: Don't hold RTNL for IPV6_LEAVE_ANYCAST and IPV6_ADDRFORM.

ipv6: anycast: Don't hold RTNL for IPV6_LEAVE_ANYCAST and IPV6_ADDRFORM.

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

In ipv6_sock_ac_drop() and ipv6_sock_ac_close(),
only __dev_get_by_index() and __in6_dev_get() requrie RTNL.

Let's replace them with dev_get_by_index() and in6_dev_get()
and drop RTNL from IPV6_LEAVE_ANYCAST and IPV6_ADDRFORM.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250702230210.3115355-13-kuni1840@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>