nbd: use loff_t for blocksize and nbd_set_size args

nbd: use loff_t for blocksize and nbd_set_size args

If we have large devices (say like the 40t drive I was trying to test with) we
will end up overflowing the int arguments to nbd_set_size and not get the right
size for our device.  Fix this by using loff_t everywhere so I don't have to
think about this again.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
[bwh: Backported to 4.9: adjust context]

Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name nbd-use-loff_t-for-blocksize-and-nbd_set_size-args.patch