repart: derive hash partition size from SizeMaxBytes= of data sibling

repart: derive hash partition size from SizeMaxBytes= of data sibling

This change makes it possible for repart to create dm-verity hash
partitions for a custom amount of protected data. When the property
`SizeMaxBytes=` is specified for a dm-verity data partition, the size
of the corresponding hash partition is set to accommodate hash data
for this maximum size, rather than the actual contents its data
sibling. However, the contained hash data continues to be generated
from said sibling.