rbd: protect against concurrent unmaps
rbd: protect against concurrent unmaps

Make sure two concurrent unmap operations on the same rbd device
won't collide, by only proceeding with the removal and cleanup of a
device if is not already underway.

Signed-off-by: Alex Elder <f429030cf5c0faf36fac3d102073b6e63a647baa@inktank.com>
Reviewed-by: Josh Durgin <51652c783f70f5b272ebd02f1fd20d3af7a2449f@inktank.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs