nvmet: don't try to add ns to p2p map unless it actually uses it

Even without CONFIG_P2PDMA this results in a error print:
nvmet: no peer-to-peer memory is available that's supported by rxe0 and /dev/nullb0

Fixes: c6925093d0b2 ("nvmet: Optionally use PCI P2P memory")
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/nvme/target/core.c

@@ -420,7 +420,7 @@ static void nvmet_p2pmem_ns_add_p2p(struct nvmet_ctrl *ctrl,
 	struct pci_dev *p2p_dev;
 	int ret;
 
-	if (!ctrl->p2p_client)
+	if (!ctrl->p2p_client || !ns->use_p2pmem)
 		return;
 
 	if (ns->p2p_dev) {