[IPV4] SNMP: Support InTruncatedPkts

An IP datagram which is being discarded because the datagram frame
didn't carry enough data should be counted as InTruncatedPkts.

Signed-off-by: Mitsuru Chinen <mitch@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>