repart: Consider existing partitions when placing

repart: Consider existing partitions when placing

Fixes an oversight in `context_allocate_partitions` that makes it
succeed in cases where it should fail. Essentially, there was nothing
actually enforcing SizeMinBytes= and PaddingMinBytes= for partitions
that exist, only for new partitions. This behavior is inconsistent with
the docs, which state that existing partitions will be grown to at least
the specified minimum size, and that "If the backing device does not
provide enough space to fulfill the constraints placing the partition
will fail".