rbd: rename rbd_id_get()

This should have been done as part of this commit:

    commit de71a297
    Author: Alex Elder <elder@inktank.com>
    Date:   Tue Jul 3 16:01:19 2012 -0500
    rbd: rename rbd_device->id

rbd_id_get() is assigning the rbd_dev->dev_id field.  Change the
name of that function as well as rbd_id_put() and rbd_id_max
to reflect what they are affecting.

Add some dynamic debug statements related to rbd device id activity.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>