libnvdimm/pfn: Account for PAGE_SIZE > info-block-size in nd_pfn_init()

Similar to "libnvdimm: Fix altmap reservation size calculation" provide
for a reservation of a full page worth of info block space at info-block
establishment time.  Typically there is already slack in the padding
from honoring the default 2MB alignment, but provide for a reservation
for corner case configurations that would otherwise fit.

Cc: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>