io_uring: allow O_NONBLOCK async retry

We can assume that O_NONBLOCK is always honored, even if we don't
have a ->read/write_iter() for the file type. Also unify the read/write
checking for allowing async punt, having the write side factoring in the
REQ_F_NOWAIT flag as well.

Cc: stable@vger.kernel.org
Fixes: 490e8967 ("io_uring: only force async punt if poll based retry can't handle it")
Signed-off-by: Jens Axboe <axboe@kernel.dk>