PCI/P2PDMA: Finish RCU conversion of pdev->p2pdma

PCI/P2PDMA: Finish RCU conversion of pdev->p2pdma

While looking at pci_alloc_p2pmem() I found RCU protection was not properly
applied there, as pdev->p2pdma was potentially read multiple times.

Fix pci_alloc_p2pmem(), add __rcu qualifier to p2pdma field of struct
pci_dev, and fix all other accesses to this field with proper RCU verbs.

Link: https://lore.kernel.org/r/20210701095621.3129283-1-eric.dumazet@gmail.com
Fixes: 1570175abd16 ("PCI/P2PDMA: track pgmap references per resource, not globally")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Jérôme Glisse" <jglisse@redhat.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>