vhost: rewind next_avail_head while discarding descriptors

vhost: rewind next_avail_head while discarding descriptors

When discarding descriptors with IN_ORDER, we should rewind
next_avail_head otherwise it would run out of sync with
last_avail_idx. This would cause driver to report
"id X is not a head".

Fixing this by returning the number of descriptors that is used for
each buffer via vhost_get_vq_desc_n() so caller can use the value
while discarding descriptors.

Fixes: 67a873df0c41 ("vhost: basic in order support")
Cc: stable@vger.kernel.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://patch.msgid.link/20251120022950.10117-1-jasowang@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>