ceph: unregister bdi before kill_anon_super releases device name

Unregister and destroy the bdi in put_super, after mount is r/o, but before
put_anon_super releases the device name.

For symmetry, bdi_destroy in destroy_client (we bdi_init in create_client).

Only set s_bdi if bdi_register succeeds, since we use it to decide whether
to bdi_unregister.

Signed-off-by: Sage Weil <sage@newdream.net>