loop: cleanup partitions when detaching loop device

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>