block: Add PCI P2P flag for request queue

Add QUEUE_FLAG_PCI_P2P, meaning a driver's request queue supports targeting
P2P memory.  This will be used by P2P providers and orchestrators (in
subsequent patches) to ensure block devices can support P2P memory before
submitting P2P-backed pages to submit_bio().

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jens Axboe <axboe@kernel.dk>