virtio-blk: Remove BUG_ON() in virtio_queue_rq()

Currently we have a BUG_ON() to make sure the number of sg
list does not exceed queue_max_segments() in virtio_queue_rq().
However, the block layer uses queue_max_discard_segments()
instead of queue_max_segments() to limit the sg list for
discard requests. So the BUG_ON() might be triggered if
virtio-blk device reports a larger value for max discard
segment than queue_max_segments(). To fix it, let's simply
remove the BUG_ON() which has become unnecessary after commit
02746e26c39e("virtio-blk: avoid preallocating big SGL for data").
And the unused vblk->sg_elems can also be removed together.

Fixes: 1f23816b8eb8 ("virtio_blk: add discard and write zeroes support")
Suggested-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Xie Yongji <[e-mail]>
Reviewed-by: Max Gurtovoy <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/block/virtio_blk.c

···
 	 */
 	refcount_t refs;
 
-	/* What host tells us, plus 2 for header & tailer. */
-	unsigned int sg_elems;
-
 	/* Ida index - used to track minor number allocations. */
 	int index;
 
···
 	bool notify = false;
 	blk_status_t status;
 	int err;
-
-	BUG_ON(req->nr_phys_segments + 2 > vblk->sg_elems);
 
 	status = virtblk_setup_cmd(vblk->vdev, req, vbr);
 	if (unlikely(status))
···
 	/* Prevent integer overflows and honor max vq size */
 	sg_elems = min_t(u32, sg_elems, VIRTIO_BLK_MAX_SG_ELEMS - 2);
 
-	/* We need extra sg elements at head and tail. */
-	sg_elems += 2;
 	vdev->priv = vblk = kmalloc(sizeof(*vblk), GFP_KERNEL);
 	if (!vblk) {
 		err = -ENOMEM;
···
 	mutex_init(&vblk->vdev_mutex);
 
 	vblk->vdev = vdev;
-	vblk->sg_elems = sg_elems;
 
 	INIT_WORK(&vblk->config_work, virtblk_config_changed_work);
 
···
 		set_disk_ro(vblk->disk, 1);
 
 	/* We can handle whatever the host told us to handle. */
-	blk_queue_max_segments(q, vblk->sg_elems-2);
+	blk_queue_max_segments(q, sg_elems);
 
 	/* No real sector limit. */
 	blk_queue_max_hw_sectors(q, -1U);
···
 		 * handled it.
 		 */
 		if (!v)
-			v = sg_elems - 2;
+			v = sg_elems;
 		blk_queue_max_discard_segments(q,
 					       min(v, MAX_DISCARD_SEGMENTS));