libnvdimm/pfn_dev: Add a build check to make sure we notice when struct page size...

libnvdimm/pfn_dev: Add a build check to make sure we notice when struct page size change

Namespaces created with PFN_MODE_PMEM mode stores struct page in the reserve
block area. We need to make sure we account for the right struct page
size while doing this. Instead of directly depending on sizeof(struct page)
which can change based on different kernel config option, use the max struct
page size (64) while calculating the reserve block area. This makes sure pmem
device can be used across kernels built with different configs.

If the above assumption of max struct page size change, we need to update the
reserve block allocation space for new namespaces created.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Link: https://lore.kernel.org/r/20190905154603.10349-4-aneesh.kumar@linux.ibm.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>