Fix bug #10096 (ceph-disk umount race condition)

Bug: http://tracker.ceph.com/issues/10096

Brief: Unmounting temporary mount point failed due to file being 'busy'.
Root cause could not be easily determined due to timing variances caused
by debug attempts. Race condition exists.

Solution: Implement a retry with incremental backoff as a viable
workaround. This workaround is okay because (1) Finding the root cause
would take a not insignificant amount of time/effort. (2) The workaround
is a more general fix for any process that might cause the exhibited
behavior.

Signed-off-by: Blaine Gardner <blaine.gardner@hp.com>