librbd: remap resize target size if encryption is loaded

librbd: remap resize target size if encryption is loaded

When encryption is loaded, rbd_get_size() and Image::size() return
"effective" size, but rbd_resize() and Image::resize() continue to take
raw size.  The user has to constantly keep these domains in mind.

Saying that resize must be done without encryption loaded is not an
answer because shrinking a clone that has snapshots involves copying up
objects in the affected range (identical to flattening).  In addition,
even if a clone doesn't have snapshots, shrinking it to a size that
isn't an object boundary is going to involve a copyup for the victim
object as well.

To avoid subtle data corruption on shrink, treat resize operation the
same as flatten operation (including on the CLI).

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