io_uring: use vmap() for ring mapping

Commit 09fc75e0 upstream.

This is the last holdout which does odd page checking, convert it to
vmap just like what is done for the non-mmap path.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>