Ceph » rbd

In http://tracker.ceph.com/issues/4183 it was reported that
unmapping an image was failing because it was busy.

Further analysis showed this was due to udev (again) triggering
"blkid" to identify the device again after the final close.
Sometimes this didn't complete before the attempt to unmap the
device, leading to the EBUSY.

The fix is to run "udevadm settle" before running the
unmap command, much like it is run after the map command.

Assigning this to Dan for now, since I believe he inserted
the "udevadm settle" command before. Dan, feel free to
reassign if you like.