PCI/P2PDMA: Annotate RCU dereference

PCI/P2PDMA: Annotate RCU dereference

A dereference of the __rcu pointer was noticed by sparse:

  drivers/pci/p2pdma.c:199:44: sparse: sparse: dereference of noderef expression

Dereference the __rcu pointer using rcu_dereference_protected() instead of
accessing it directly. It's safe to use rcu_dereference_protected() because
a reference is held on the pgmap's percpu reference counter and thus it
cannot disappear.

Link: https://lore.kernel.org/r/20230209172953.4597-1-logang@deltatee.com
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>