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>
(cherry picked from commit 235b27a8f08a82fc46efa8b6a3e5cb8a5276956b)