virtio_net: Split struct virtio_net_rss_config

struct virtio_net_rss_config was less useful in actual code because of a
flexible array placed in the middle. Add new structures that split it
into two to avoid having a flexible array in the middle.

Suggested-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Link: https://patch.msgid.link/20250321-virtio-v2-1-33afb8f4640b@daynix.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>