ext2: don't overflow when creating a partition of size 2TiB or larger

ext2: don't overflow when creating a partition of size 2TiB or larger

Without this patch, mkpartfs would succeed in creating an ext2
partition, but the resulting partition would fail a mkfs.ext2 -f check.
* libparted/fs/ext2/parted_io.c (do_read): Avoid overflow 32-bit
sector count overflow for a partition of size 2TiB or larger.
(do_write): Likewise.
See http://bugzilla.redhat.com/584057#c16 a reproducer.