net: ipv6: add net argument to ip6_dst_lookup_flow

mainline inclusion
from mainline-v5.5-rc1
commit c4e85f73
category: bugfix
bugzilla: 13690
CVE: CVE-2020-1749

It's prepare for fixing CVE-2020-1749.

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

This will be used in the conversion of ipv6_stub to ip6_dst_lookup_flow,
as some modules currently pass a net argument without a socket to
ip6_dst_lookup. This is equivalent to commit 343d60aa ("ipv6: change
ipv6_stub_impl.ipv6_dst_lookup to take net argument").

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Wenan Mao <maowenan@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>