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: c6925093

 ("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>