A helper is introduced for the logic of allocating the descriptor
extra data. This will be reused by split virtqueue.
Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210604055350.58753-4-jasowang@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>