ipv6: slight optimization in ip6_dst_gc

entries is always greater than rt_max_size here, since if entries is less
than rt_max_size, the fib6_run_gc function will be skipped

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>