rbd: switch to ida for rbd id assignments

Currently rbd ids are allocated using an atomic variable that keeps
track of the highest id currently in use and each new id is simply one
more than the value of that variable.  That's nice and cheap, but it
does mean that rbd ids are allowed to grow boundlessly, and, more
importantly, it's completely unpredictable.  So, in preparation for
single-major device number allocation scheme, which is going to
establish and rely on a constant mapping between rbd ids and device
numbers, switch to ida for rbd id assignments.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>