ipv6: Do not drop packet if skb->local_df is set to true

The old code will drop IPv6 packet if ipfragok is not set, since
ipfragok is obsoleted, will be instead by used skb->local_df, so this
check must be changed to skb->local_df.

This patch fix this problem and not drop packet if skb->local_df is
set to true.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>