rbd: a few simple changes

Here are a few very simple cleanups:
    - Add a "RBD_" prefix to the two driver name string definitions.
    - Move the definition of struct rbd_request below struct rbd_req_coll
      to avoid the need for an empty declaration of the latter.
    - Move and group the definitions of rbd_root_dev_release() and
      rbd_root_dev, as well as rbd_bus_type and rbd_bus_attrs[],
      close to the top of the file.  Arrange the latter so
      rbd_bus_type.bus_attrs can be initialized statically.
    - Get rid of an unnecessary local variable in rbd_open().
    - Rework some hokey logic in rbd_bus_add_dev(), so the value of
      "ret" at the end is either 0 or -ENOENT to avoid the need for
      the code duplication that was there.
    - Rename a goto target in rbd_add().

Signed-off-by: Alex Elder <elder@dreamhost.com>
Signed-off-by: Sage Weil <sage@newdream.net>