libnvdimm, pfn: make 'resource' attribute only readable by root

commit 26417ae4 upstream.

For the same reason that /proc/iomem returns 0's for non-root readers
and acpi tables are root-only, make the 'resource' attribute for pfn
devices only readable by root. Otherwise we disclose physical address
information.

Fixes: f6ed58c7 ("libnvdimm, pfn: 'resource'-address and 'size'...")
Reported-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>