libnvdimm, pfn: 'struct page' provider infrastructure

Implement the base infrastructure for libnvdimm PFN devices. Similar to
BTT devices they take a namespace as a backing device and layer
functionality on top. In this case the functionality is reserving space
for an array of 'struct page' entries to be handed out through
pfn_to_page(). For now this is just the basic libnvdimm-device-model for
configuring the base PFN device.

As the namespace claiming mechanism for PFN devices is mostly identical
to BTT devices drivers/nvdimm/claim.c is created to house the common
bits.

Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/nvdimm/Kconfig

@@ -21,6 +21,7 @@ config BLK_DEV_PMEM
 	default LIBNVDIMM
 	depends on HAS_IOMEM
 	select ND_BTT if BTT
+	select ND_PFN if NVDIMM_PFN
 	help
 	  Memory ranges for PMEM are described by either an NFIT
 	  (NVDIMM Firmware Interface Table, see CONFIG_NFIT_ACPI), a
@@ -47,12 +48,16 @@ config ND_BLK
 	  (CONFIG_ACPI_NFIT), or otherwise exposes BLK-mode
 	  capabilities.
 
+config ND_CLAIM
+	bool
+
 config ND_BTT
 	tristate
 
 config BTT
 	bool "BTT: Block Translation Table (atomic sector updates)"
 	default y if LIBNVDIMM
+	select ND_CLAIM
 	help
 	  The Block Translation Table (BTT) provides atomic sector
 	  update semantics for persistent memory devices, so that
@@ -65,4 +70,21 @@ config BTT
 
 	  Select Y if unsure
 
+config ND_PFN
+	tristate
+
+config NVDIMM_PFN
+	bool "PFN: Map persistent (device) memory"
+	default LIBNVDIMM
+	select ND_CLAIM
+	help
+	  Map persistent memory, i.e. advertise it to the memory
+	  management sub-system.  By default persistent memory does
+	  not support direct I/O, RDMA, or any other usage that
+	  requires a 'struct page' to mediate an I/O request.  This
+	  driver allocates and initializes the infrastructure needed
+	  to support those use cases.
+
+	  Select Y if unsure
+
 endif