ipv6: Add __in6_dev_get_rtnl_net().

ipv6: Add __in6_dev_get_rtnl_net().

We will convert rtnl_lock() with rtnl_net_lock(), and we want to
convert __in6_dev_get() too.

__in6_dev_get() uses rcu_dereference_rtnl(), but as written in its
comment, rtnl_dereference() or rcu_dereference() is preferable.

Let's add __in6_dev_get_rtnl_net() that uses rtnl_net_dereference().

We can add the RCU version helper later if needed.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250115080608.28127-2-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>