rbd: fix bug in rbd_dev_id_put()

In rbd_dev_id_put(), there's a loop that's intended to determine
the maximum device id in use.  But it isn't doing that at all,
the effect of how it's written is to simply use the just-put id
number, which ignores whole purpose of this function.

Fix the bug.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
(cherry picked from commit b213e0b1a62637b2a9395a34349b13d73ca2b90a)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>