CVE-2025-38413: 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 buftoxdp, the len argument is the frame data's length without virtio header's length (vi->hdrlen). We check that len with

xskpoolgetrxframesize() + vi->hdrlen

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

hardstart + XDPPACKETHEADROOM - vi->hdrlen not hardstart + XDPPACKETHEADROOM

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

xskpoolgetrxframesize() not xskpoolgetrxframesize() + vi->hdrlen

like in the current check.

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