ipv6/route: return if there is no fib_nh_gw_family

Previously we will return directly if (!rt || !rt->fib6_nh.fib_nh_gw_family)
in function rt6_probe(), but after commit cc3a86c8
("ipv6: Change rt6_probe to take a fib6_nh"), the logic changed to
return if there is fib_nh_gw_family.

Fixes: cc3a86c8 ("ipv6: Change rt6_probe to take a fib6_nh")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>