block: export 'ro' sysfs attribute for partitions

We already export 'ro' for the disk. This adds the same attribute
for partitions.

Cc: Karel Zak <kzak@redhat.com>
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>