Currently we set them to 512 bytes unless manually specified.  Unforuntaly
some brain-dead partitioning tools create unaligned partitions if they
get low enough optiomal I/O size values, so don't report any at all
unless explicitly set.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>