nvme/io_uring: use helper for polled completions

NVMe is making up issue_flags, which is a no-no in general, and to make
matters worse, they are completely the wrong ones. For a pure polled
request, which it does check for, we're already inside the
ctx->uring_lock when the completions are run off io_do_iopoll(). Hence
the correct flag would be '0' rather than IO_URING_F_UNLOCKED.

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