net: conn: Add proper processing of DGRAM packet sockets

Datagram AF_PACKET sockets were not processed properly by the net stack.
Instead of receving a packet already processed L2, and thus with L2
header trimmed, it was receiving a raw, unprocessed packet.

Fix this by calling net_packet_socket_input() for the second time, after
L2 has processed the packet. An updated connection handler module will
forward the packet correctly based on the corresponding socket type and
packet L2 processing status.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>