rbd: simplify snap_by_name() interface

There is only one caller of snap_by_name(), and it passes two values
to be assigned, both of which are found within an rbd device
structure.

Change the interface so it just passes the address of the rbd_dev,
and make the assignments to its fields directly.

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