librbd: introduce reduce_parent_overlap() and switch overlap API

librbd: introduce reduce_parent_overlap() and switch overlap API

When encryption is loaded, rbd_get_overlap() and Image::overlap() now
return "effective" overlap, similar to rbd_get_size() and Image::size().
Previously, returned overlap could have been bigger than "effective"
size.

Note that get_effective_image_size() successor doesn't take snap_id
because passing anything but ictx->snap_id was broken.  Since the size
of the crypto header is stored in the crypto header itself, image areas
are defined only for the "opened at" snap_id.  Getting "effective" size
for an arbitrary snapshot requires actually opening it and loading
encryption on it.

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