Physically-contiguous buffers

Not only that, but you could also pre-map the SG lists, instead of having to do map SG and unmap SG for each IO. Right now the registered buffers only avoid the get_user_pages() and put_pages() for each IO, which is (by far) the biggest overhead. But if we fix the kernel parts as well, then we can avoid the dma map/unmap for each IO. That'd bypass the split as well, some quick mental math shows we should be able to kill ~5% of the overhead on my box with that.

In general we have various pieces of low hanging fruit on the block layer side, which are readily apparent now that we have an efficient interface into the kernel. Work in progress! But I'd like to wrap up io_uring first.