rbd: add force close option

This adds a force close option, so we can force the unmapping
of a rbd device that is open. If a path/device is blacklisted, apps
like multipathd can map a new device and then unmap the old one.
The unmapping cleanup would then be handled by the generic hotunplug
code paths in multipahd like is done for iSCSI, FC/FCOE, SAS, etc.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>