NBD_REP_ERR_INVALID is often parameter to nbd_opt_drop and it would
be used more in following patches. So, let's add a helper.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <20180312152126.286890-2-vsementsov@virtuozzo.com>
Signed-off-by: NEric Blake <eblake@redhat.com>