libnvdimm, pfn: fix start_pad handling for aligned namespaces

The alignment checks at pfn driver startup fail to properly account for
the 'start_pad' in the case where the namespace is misaligned relative
to its internal alignment. This is typically triggered in 1G aligned
namespace, but could theoretically trigger with small namespace
alignments. When this triggers the kernel reports messages of the form:

    dax2.1: bad offset: 0x3c000000 dax disabled align: 0x40000000

Cc: <stable@vger.kernel.org>
Fixes: 1ee6667c

 ("libnvdimm, pfn, dax: fix initialization vs autodetect...")
Reported-by: Jane Chu <jane.chu@oracle.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>