rbd: use the correct length for format 2 object names

Format 2 objects use 16 characters for the object name suffix to be
able to express the full 64-bit range of object numbers. Format 1
images only use 12 characters for this. Using 12-character names for
format 2 caused userspace and kernel rbd clients to read differently
named objects, which made an image written by one client look empty to
the other client.

Reported-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>