PCI/P2PDMA: Fix build without DMA ops

My commit to make DMA ops support optional missed the reference in
the p2pdma code.  And while the build bot didn't manage to find a config
where this can happen, Matthew did.  Fix this by replacing two IS_ENABLED
checks with ifdefs.

Fixes: 2f9237d ("dma-mapping: make support for dma ops optional")
Link: https://lore.kernel.org/r/20200810124843.1532738-1-hch@lst.de
Reported-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>

Author: Christoph Hellwig

drivers/pci/p2pdma.c

@@ -556,13 +556,14 @@ int pci_p2pdma_distance_many(struct pci_dev *provider, struct device **clients,
 		return -1;
 
 	for (i = 0; i < num_clients; i++) {
-		if (IS_ENABLED(CONFIG_DMA_VIRT_OPS) &&
-		    clients[i]->dma_ops == &dma_virt_ops) {
+#ifdef CONFIG_DMA_VIRT_OPS
+		if (clients[i]->dma_ops == &dma_virt_ops) {
 			if (verbose)
 				dev_warn(clients[i],
 					 "cannot be used for peer-to-peer DMA because the driver makes use of dma_virt_ops\n");
 			return -1;
 		}
+#endif
 
 		pci_client = find_parent_pci_dev(clients[i]);
 		if (!pci_client) {
@@ -842,9 +843,10 @@ static int __pci_p2pdma_map_sg(struct pci_p2pdma_pagemap *p2p_pgmap,
 	 * this should never happen because it will be prevented
 	 * by the check in pci_p2pdma_distance_many()
 	 */
-	if (WARN_ON_ONCE(IS_ENABLED(CONFIG_DMA_VIRT_OPS) &&
-			 dev->dma_ops == &dma_virt_ops))
+#ifdef CONFIG_DMA_VIRT_OPS
+	if (WARN_ON_ONCE(dev->dma_ops == &dma_virt_ops))
 		return 0;
+#endif
 
 	for_each_sg(sg, s, nents, i) {
 		paddr = sg_phys(s);