librbd/deep_copy: skip snap list if object is known to be clean

librbd/deep_copy: skip snap list if object is known to be clean

If the fast-diff indicates that the destination object should exist
and that it hasn't changed, there shouldn't be a need to issue the
snap list operation. Instead, just update the destination object map
to indicate the existence of the object.

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