IB/qib: check for user backed iterator, not specific iterator type

In preparation for switching single segment iterators to using ITER_UBUF,
swap the check for whether we are user backed or not.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/infiniband/hw/qib/qib_file_ops.c

@@ -2245,7 +2245,7 @@ static ssize_t qib_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	struct qib_ctxtdata *rcd = ctxt_fp(iocb->ki_filp);
 	struct qib_user_sdma_queue *pq = fp->pq;
 
-	if (!iter_is_iovec(from) || !from->nr_segs || !pq)
+	if (!from->user_backed || !from->nr_segs || !pq)
 		return -EINVAL;
 
 	return qib_user_sdma_writev(rcd, pq, iter_iov(from), from->nr_segs);