rbd: split rbd_dev_spec_update() into two functions

rbd_dev_spec_update() has two modes of operation, with nothing in
common between them.  Split it into two functions, one for each mode
and make our expectations more clear.

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