Rather than use remap_pfn_range() for this and manually free later,
switch to using vm_insert_pages() and have it Just Work.

If possible, allocate a single compound page that covers the range that
is needed. If that works, then we can just use page_address() on that
page. If we fail to get a compound page, allocate single pages and use
vmap() to map them into the kernel virtual address space.

This just covers the rings/sqes, the other remaining user of the mmap
remap_pfn_range() user will be converted separately. Once that is done,
we can kill the old alloc/free code.

Signed-off-by: Jens Axboe <axboe@kernel.dk>