rbd: whole-object write and zeroout should copyup when snapshots exist

Otherwise, once the parent snapshot is removed, the clone's snapshot
wouldn't reflect the state of the clone prior to whole-object write or
zeroout because a deep-copyup was never done ("rbd flatten" wouldn't do
it because the modified object would exist in HEAD).

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>