net: conn: Check UDP and TCP checksum before accepting packet

We did not check UDP or TCP checksum to be valid after receiving
the packet. Fix this so that the checksum is validated when
packet is received in connection handler. As the checksum validation
can be resource intensive, do it after we have verified that
there is a connection handler for this connection.

The checksum calculation can be turned OFF if needed, but it is
ON by default.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>