io_uring: simplify io_file_get()

Keep ->needs_file_no_error check out of io_file_get(), and let callers
handle it. It makes it more straightforward. Also, as the only error it
can hand back -EBADF, make it return a file or NULL.

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