virtio-balloon: Avoid using the word 'report' when referring to free page hinting

It can be confusing to have multiple features within the same driver that
are using the same verbage. As such this patch is creating a union of
free_page_report_cmd_id with free_page_hint_cmd_id so that we can clean-up
the userspace code a bit in terms of readability while maintaining the
functionality of legacy code.
Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Link: https://lore.kernel.org/r/20200415174318.13597.99753.stgit@localhost.localdomainSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_balloon.c

@@ -580,7 +580,7 @@ static u32 virtio_balloon_cmd_id_received(struct virtio_balloon *vb)
 	if (test_and_clear_bit(VIRTIO_BALLOON_CONFIG_READ_CMD_ID,
 			       &vb->config_read_bitmap))
 		virtio_cread(vb->vdev, struct virtio_balloon_config,
-			     free_page_report_cmd_id,
+			     free_page_hint_cmd_id,
 			     &vb->cmd_id_received_cache);
 
 	return vb->cmd_id_received_cache;

include/uapi/linux/virtio_balloon.h

@@ -48,8 +48,15 @@ struct virtio_balloon_config {
 	__u32 num_pages;
 	/* Number of pages we've actually got in balloon. */
 	__u32 actual;
-	/* Free page report command id, readonly by guest */
-	__u32 free_page_report_cmd_id;
+	/*
+	 * Free page hint command id, readonly by guest.
+	 * Was previously named free_page_report_cmd_id so we
+	 * need to carry that name for legacy support.
+	 */
+	union {
+		__u32 free_page_hint_cmd_id;
+		__u32 free_page_report_cmd_id;	/* deprecated */
+	};
 	/* Stores PAGE_POISON if page poisoning is in use */
 	__u32 poison_val;
 };