[Bug] DAG pieces get padded to full sector size

[parkan]

when a DAG piece is produced it's specified as full sector size despite being data size being very small:

baga6ea4seaqgmowdulb5xtlwmxsmedeuggznygn45evetj5ul3ut7sotpdaqeei  34359738368  bafybeihxmwrn7kll74iialjcetonrpw3z4vuorhgf5oe7xcoo3hwqusoxi  285012

this is likely not going to be accepted by SPs, similarly the last piece of a prep (under-full chunk) appears to get padded to full sector

we will need to either change how the writer behaves with these pieces or come up with a workaround