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

In the Linux kernel, the following vulnerability has been resolved:

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.