ipv6: route: enforce RCU protection in ip6_route_check_nh_onlink()

mainline inclusion
from mainline-v5.0-rc7
commit bf1dc8ba
category: bugfix
bugzilla: 10918
CVE: NA

-------------------------------------------------

We need a RCU critical section around rt6_info->from deference, and
proper annotation.

Fixes: 4ed591c8 ("net/ipv6: Allow onlink routes to have a device mismatch if it is the default route")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Lin Miaohe <linmiaohe@huawei.com>
Reviewed-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>