[IPv6]: Exclude truncated packets from InHdrErrors statistics

Incoming trancated packets are counted as not only InTruncatedPkts but
also InHdrErrors. They should be counted as InTruncatedPkts only.

Signed-off-by: Mitsuru Chinen <mitch@linux.vnet.ibm.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>