librbd: new hook for pre-processing copyup data

librbd: new hook for pre-processing copyup data

This will permit the crypto layer to properly encrypt and potentially
align the sparse copyup data prior to it being written. It passes
potentially multiple sets of data in one pass to permit the deep-copy
state machine to utilize the same API and allow the crypto layer to
potentially handle layered alignment issues.

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