nbd/server: fix nbd_co_send_block_status

Call nbd_co_send_extents() with correct length parameter
(extent.length may be smaller than original length).

Also, switch length parameter type to uint32_t, to correspond with
request->len and similar nbd_co_send_bitmap().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180704112302.471456-2-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>