rtnetlink: Factorise do_setlink() path from __rtnl_newlink().

__rtnl_newlink() got too long to maintain.

For example, netdev_master_upper_dev_get()->rtnl_link_ops is fetched even
when IFLA_INFO_SLAVE_DATA is not specified.

Let's factorise the single dev do_setlink() path to a separate function.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>