iov_iter: move iter_ubuf check inside restore WARN

io_uring is using iter_ubuf types for single vector requests. We expect
state restore may happen for this type now, and it is already handled
correctly, so suppress the warning.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Christoph Hellwig <hch@lst.de>