rbd: restore zeroing past the overlap when reading from parent

The parent image is read only up to the overlap point, the rest of
the buffer should be zeroed.  This snuck in because as it turns out
the overlap test case has not been triggering this code path for
a while now.

Fixes: a9b67e69 ("rbd: replace obj_req->tried_parent with obj_req->read_state")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>