ipv6: Handle all fib6_nh in a nexthop in rt6_nlmsg_size

Add a hook in rt6_nlmsg_size to handle nexthop struct in a fib6_info.
rt6_nh_nlmsg_size is used to sum the space needed for all nexthops in
the fib entry.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>