io_uring: add support for IORING_OP_OPENAT2

Add support for the new openat2(2) system call. It's trivial to do, as
we can have openat(2) just be wrapped around it.

Suggested-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>