io_uring: deduplicate io_openat{,2}_prep()

io_openat_prep() and io_openat2_prep() are identical except for how
struct open_how is built. Deduplicate it with a helper.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>