io_uring: fix openat/openat2 unified prep handling

A previous commit unified how we handle prep for these two functions,
but this means that we check the allowed context (SQPOLL, specifically)
later than we should. Move the ring type checking into the two parent
functions, instead of doing it after we've done some setup work.

Fixes: ec65fea5 ("io_uring: deduplicate io_openat{,2}_prep()")
Reported-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>