librbd: support preprocessing parent data prior to copyup

librbd: support preprocessing parent data prior to copyup

Let object dispatch layers potentially mutate the copyup data read
from the parent prior to issuing the actual copyup operation. This
can allow for a layer like the crypto layer to re-encrypt the parent
image data using the child image's encryption keys, for example.

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