zebra: fix rtnh_len in the rt_netlink messages for multipath case

In _netlink_route_build_multipath():
- Each time when appending a IPv4 gateway in the message, rtnh_len
  is increased by sizeof (struct rtattr) + 4, where we should use
  "bytelen" instead of the hard coding "4".
- As what done for IPv4, we should increase rtnh_len accordingly
  along with adding a IPv6 gateway, or else the IPv6 gateways will
  be lost.

Signed-off-by: Feng Lu <lu.feng@6wind.com>
Reviewed-by: Alain Ritoux <alain.ritoux@6wind.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

zebra/rt_netlink.c

@@ -1561,7 +1561,7 @@ _netlink_route_build_multipath(
     {
       rta_addattr_l (rta, NL_PKT_BUF_SIZE, RTA_GATEWAY,
                      &nexthop->gate.ipv4, bytelen);
-      rtnh->rtnh_len += sizeof (struct rtattr) + 4;
+      rtnh->rtnh_len += sizeof (struct rtattr) + bytelen;
 
       if (nexthop->src.ipv4.s_addr)
         *src = &nexthop->src;
@@ -1580,6 +1580,7 @@ _netlink_route_build_multipath(
     {
       rta_addattr_l (rta, NL_PKT_BUF_SIZE, RTA_GATEWAY,
                      &nexthop->gate.ipv6, bytelen);
+      rtnh->rtnh_len += sizeof (struct rtattr) + bytelen;
 
       if (IS_ZEBRA_DEBUG_KERNEL)
         zlog_debug("netlink_route_multipath() (%s): "