use int64_t for return values from rbd instead of int

use int64_t for return values from rbd instead of int

rbd / rados tends to return pretty often length of writes
or discarded blocks. These values might be bigger than int.

The steps to reproduce are:

  mkfs.xfs -f a whole device bigger than int in bytes. mkfs.xfs sends
  a discard. Important is that you use scsi-hd and set
  discard_granularity=512. Otherwise rbd disabled discard support.

Signed-off-by: Stefan Priebe <s.priebe@profihost.ag>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>