Linux usbfs zero-copy support

[martinling]

Since Linux 4.9, the usbfs subsystem includes a mechanism to support zero-copy transfers, by allowing userspace to mmap an open usbfs device to get a DMA-ready buffer. When such a buffer is used in a request, the kernel can arrange for the host controller to access it directly.

This feature is supported in libusb via the libusb_dev_mem_alloc and libusb_dev_mem_free functions.

It would be nice if nusb could use this feature, as it may make a significant performance difference especially at SuperSpeed, but there would be some API and safety implications.

In principle, you could add a method to Device which returns a RequestBuffer allocated this way with mmap, and put some metadata in the RequestBuffer struct to indicate that drop needs to call munmap. However, it would then no longer be safe to turn that RequestBuffer into a normal Vec and back.