Provide more headroom for packets to allow `bpf_xdp_adjust_head` to be used

I am trying to deploy an XDP program to GCP with gVNIC that uses `bpf_xdp_adjust_head`. This currently fails in mysterious ways and after some debugging and reading the source code, I think I have figured out why:

The driver only provides us with two bytes of headroom:

https://github.com/GoogleCloudPlatform/compute-virtual-ethernet-linux/blob/d5ad781d87806deb507a9760d6b11a9ab4144eca/google/gve/gve_rx.c#L858-L860

The constant passed here, `GVE_RX_PAD` is defined as 2:

https://github.com/GoogleCloudPlatform/compute-virtual-ethernet-linux/blob/d5ad781d87806deb507a9760d6b11a9ab4144eca/build/gve_desc.h#L86

That is unusually low.

The Linux kernel itself defines the headroom as 256:

https://github.com/torvalds/linux/blob/94d471a4f42862bf53dc3776bde4b4c9175acbd4/include/uapi/linux/bpf.h#L6449

Would it be possible to adjust this? If I compile the driver myself (which this constant changed), are things "just" going to work or are there more things that need to be changed?