net/ipv6: Handle onlink flag with multipath routes

For multipath routes the ONLINK flag can be specified per nexthop in
rtnh_flags or globally in rtm_flags. Update ip6_route_multipath_add
to consider the ONLINK setting coming from rtnh_flags. Each loop over
nexthops the config for the sibling route is initialized to the global
config and then per nexthop settings overlayed. The flag is 'or'ed into
fib6_config to handle the ONLINK flag coming from either rtm_flags or
rtnh_flags.

Fixes: fc1e64e1 ("net/ipv6: Add support for onlink flag")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -4112,6 +4112,7 @@ static int ip6_route_multipath_add(struct fib6_config *cfg,
 				r_cfg.fc_encap_type = nla_get_u16(nla);
 		}
 
+		r_cfg.fc_flags |= (rtnh->rtnh_flags & RTNH_F_ONLINK);
 		rt = ip6_route_info_create(&r_cfg, extack);
 		if (IS_ERR(rt)) {
 			err = PTR_ERR(rt);