zebra: fix rtnh_len in the rt_netlink messages for multipath case

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>
(cherry picked from commit 621e2aaf33d8ab73bf44b0eea3f3900135d34996)

Conflicts:
	zebra/rt_netlink.c