io_uring: add mapping support for NOMMU archs

That is a bit weird scenario but I find it interesting to run fio loads
using LKL linux, where MMU is disabled.  Probably other real archs which
run uClinux can also benefit from this patch.

Signed-off-by: Roman Penyaev <rpenyaev@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>