nbd: set the logical and physical blocksize properly

We noticed when trying to do O_DIRECT to an export on the server side
that we were getting requests smaller than the 4k sectorsize of the
device.  This is because the client isn't setting the logical and
physical blocksizes properly for the underlying device.  Fix this up by
setting the queue blocksizes and then calling bd_set_size.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>