virtio-net: xsk: rx: fix the frame's length check

When calling buf_to_xdp, the len argument is the frame data's length
without virtio header's length (vi->hdr_len). We check that len with

	xsk_pool_get_rx_frame_size() + vi->hdr_len

to ensure the provided len does not larger than the allocated chunk
size. The additional vi->hdr_len is because in virtnet_add_recvbuf_xsk,
we use part of XDP_PACKET_HEADROOM for virtio header and ask the vhost
to start placing data from

	hard_start + XDP_PACKET_HEADROOM - vi->hdr_len
not
	hard_start + XDP_PACKET_HEADROOM

But the first buffer has virtio_header, so the maximum frame's length in
the first buffer can only be

	xsk_pool_get_rx_frame_size()
not
	xsk_pool_get_rx_frame_size() + vi->hdr_len

like in the current check.

This commit adds an additional argument to buf_to_xdp differentiate
between the first buffer and other ones to correctly calculate the maximum
frame's length.

Cc: stable@vger.kernel.org
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Fixes: a4e7ba70 ("virtio_net: xsk: rx: support recv small mode")
Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
Link: https://patch.msgid.link/20250630151315.86722-2-minhquangbui99@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>