Instead of a variable 'harderror' we can simply try to correctly
propagate errors to the userspace.

This patch removes the harderror variable and passes errors through
error pointers and nbd_do_it back to the userspace.
Signed-off-by: NMarkus Pargmann <mpa@pengutronix.de>
Acked-by: NPavel Machek <pavel@ucw.cz>
Signed-off-by: NJens Axboe <axboe@fb.com>