nbd-vram: Use your NVIDIA GPU's VRAM as swap space on Linux

The "obvious" approach is nvidia_p2p_get_pages_persistent, which pins VRAM pages in BAR1 so the CPU can access them directly via ioremap_wc. Every existing project that tried this route hits the same wall: the NVIDIA driver returns EINVAL on consumer GeForce GPUs. Both the persistent and non-persistent variants, both flag values. It's gated at the RM level for Quadro/datacenter SKUs only, regardless of driver version.

The other approach - directly ioremap_wc the BAR1 physical address without going through the P2P API - also doesn't work. The GPU's internal page tables only have ~16 MiB of BAR1 mapped (just the display framebuffer). Reads from the rest return zeros. mkswap appears to succeed, then swapon fails because the swap header isn't actually there.

The NBD approach sidesteps all of this. cuMemcpyHtoD and cuMemcpyDtoH work on any CUDA GPU without any special permissions.

I thought it was interesting to see a glimpse of how Nvidia gates some features to certain models.