virtio-net: Fix num_buffers for version 1

virtio-net: Fix num_buffers for version 1

The specification says the device MUST set num_buffers to 1 if
VIRTIO_NET_F_MRG_RXBUF has not been negotiated.

Fixes: df91055db5c9 ("virtio-net: enable virtio 1.0")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20250108-buffers-v1-1-a0c85ff31aeb@daynix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit c17ad4b11bd268a35506cd976884562df6ca69d7)
(Mjt: adjust for 7.2.x)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>