bridge: br_arp_nd_proxy: set icmp6_router if neigh has NTF_ROUTER

mainline inclusion
from mainline-4.20
commit 7aca011f
category: bugfix
bugzilla: 6062
CVE: NA

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

Fixes: ed842fae ("bridge: suppress nd pkts on BR_NEIGH_SUPPRESS ports")
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Keefe LIU <liuqifa@huawei.com>
Reviewed-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>