Any partitions added by user space to the loop device were being
left in place after detaching the loop device.  This was because
the detach path issued a BLKRRPART to clean up partitions if
LO_FLAGS_PARTSCAN was set, meaning that the partitions were auto
scanned on attach.  Replace this BLKRRPART with code that
unconditionally cleans up partitions on detach instead.
Signed-off-by: Phillip Susi <psusi@ubuntu.com>

Modified by Jens to export delete_partition().
Signed-off-by: Jens Axboe <axboe@kernel.dk>