pnio_dcp: compute block length and padding when building response blocks

[bIackr0se]

Description

Building PROFINET DCP response blocks writes wrong block lengths and stray padding bytes:

• DCPIPBlock, DCPFullIPBlock, DCPDeviceIDBlock, DCPDeviceOptionsBlock and DCPOEMIDBlock use LenField("dcp_block_length", None), which counts the payload. A block alone gets length 0, and a block stacked before other blocks gets their length.
• DCPMACBlock and DCPManufacturerSpecificBlock use FieldLenField without length_of, so building raises Field should have either length_of or count_of.
• DCPDeviceInitiativeBlock leaves BlockInfo out of its length (2 instead of 4).
• The padding default b"\x00" is written after every block, also when the block length is even.

p = ProfinetDCP(service_id=DCP_SERVICE_ID_IDENTIFY, service_type=DCP_RESPONSE) / \
    DCPNameOfStationBlock(name_of_station="plc-1") / \
    DCPIPBlock(ip="192.168.0.10") / DCPDeviceIDBlock() / DCPDeviceRoleBlock()

Before, the IP block has length 20 and is followed by a zero byte, and Wireshark decodes the IP block and then reports [Malformed Packet: PN-DCP]. After, the block lengths are 7, 14, 6 and 4, the only padding byte follows the odd-length NameOfStation block, and Wireshark decodes all four blocks.

Fixed-size blocks now default to their constant length, as DCPDeviceRoleBlock already does, and variable-size blocks use FieldLenField with length_of, as DCPNameOfStationBlock does. padding defaults to b"" and is written as one zero byte only after an odd-length block; a value set by hand is cut or zero-filled to that length. DCPFullIPBlock now defaults to four DNS addresses, the number its dissector reads. Dissection is unchanged.

This message was written with the help of AI (Claude Opus 5.5).