libnvdimm, pmem: 'struct page' for pmem

libnvdimm, pmem: 'struct page' for pmem

Orabug: 22913653

Enable the pmem driver to handle PFN device instances.  Attaching a pmem
namespace to a pfn device triggers the driver to allocate and initialize
struct page entries for pmem.  Memory capacity for this allocation comes
exclusively from RAM for now which is suitable for low PMEM to RAM
ratios.  This mechanism will be expanded later for setting an "allocate
from PMEM" policy.

Note: I've renamed the parameter "size" of routine
"pmem_direct_access()" to "not_used".  This parameter was not being used
by any caller, so upstream removed it, but UEK needed to retain it to
maintain kABI compatibility.  In this commit, though, upstream added
a local variable called "size", which caused a name conflict with the
parameter.  - ldd

Cc: Boaz Harrosh <boaz@plexistor.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
(cherry picked from commit 32ab0a3f51701cb37ab960635254d5f84ec3de0a)
Signed-off-by: Dan Duval <dan.duval@oracle.com>