virtio_ring: fix typos in vring_desc_extra

We're actually tracking descriptor address and length instead of the
buffer.

Signed-off-by: Jason Wang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/virtio/virtio_ring.c

···
 };
 
 struct vring_desc_extra {
-	dma_addr_t addr;		/* Buffer DMA addr. */
-	u32 len;			/* Buffer length. */
+	dma_addr_t addr;		/* Descriptor DMA addr. */
+	u32 len;			/* Descriptor length. */
 	u16 flags;			/* Descriptor flags. */
 	u16 next;			/* The next desc state in a list. */
 };