librbd/io: track object non-existence when computing snapshot deltas

librbd/io: track object non-existence when computing snapshot deltas

Re-use the existing DNE state to track whether or not the object
already exists when computing snapshot deltas from an arbitrary
set of snapshots. Previously, the non-existence of the object was
only computed for snap id 0 for tracking whiteouts. In a future
commit, the deep-copy object-copy state machine will be able to
properly update the object-map state to indicate exists clean
vs non-existent state.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>