krbd: rework the unmap retry loop

The retry loop in the unmap path turned out to be insufficient for
doing long fsx -K runs.  Replace it with a single retry and then call
out to 'udevadm settle' if the retry doesn't help.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #2040 from ceph/wip-krbd-settle

krbd: rework the unmap retry loop

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>