luks: extract qcrypto_block_calculate_payload_offset()

luks: extract qcrypto_block_calculate_payload_offset()

The qcow2 .bdrv_measure() code calculates the crypto payload offset.
This logic really belongs in crypto/block.c where it can be reused by
other image formats.

The "luks" block driver will need this same logic in order to implement
.bdrv_measure(), so extract the qcrypto_block_calculate_payload_offset()
function now.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-Id: <20200221112522.1497712[email protected]>
Signed-off-by: Max Reitz <[email protected]>