nbd: set the max segment size to UINT_MAX

NBD doesn't care about limiting the segment size, let the user push the
largest bio's they want.  This allows us to control the request size
solely through max_sectors_kb.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>