Allow rdma_delete_block to work without the hash

In the next patch we remove the hash on the destination,
rdma_delete_block does two things with the hash which can be avoided:
  a) The caller passes the offset and rdma_delete_block looks it up
     in the hash; fixed by getting the caller to pass the block
  b) The hash gets recreated after deletion; fixed by making that
     conditional on the hash being initialised.

While this function is currently only used during cleanup, Michael
asked that we keep it general for future dynamic block registration
work.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>