virtio-blk: limit seg_max to a safe value

The struct virtio_blk_config seg_max value is read from the device and
incremented by 2 to account for the request header and status byte
descriptors added by the driver.

In preparation for supporting untrusted virtio-blk devices, protect
against integer overflow and limit the value to a safe maximum.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20210524154020.98195-1-stefanha@redhat.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>