ipv6: avoid a dst_entry refcount change in ipv6_destopt_rcv()

ipv6_destopt_rcv() runs with rcu_read_lock(), so there is no need to
take a temporay reference on dst_entry, even if skb is freed by
ip6_parse_tlv()

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>