libnvdimm, pfn: during init, clear errors in the metadata area

If there are badblocks present in the 'struct page' area for pfn
namespaces, until now, the only way to clear them has been to force the
namespace into raw mode, clear the errors, and re-enable the fsdax mode.
This is clunky, given that it should be easy enough for the pfn driver
to do the same.

Add a new helper that uses the most recently available badblocks list to
check whether there are any badblocks that lie in the volatile struct
page area. If so, before initializing the struct pages, send down
targeted writes via nvdimm_write_bytes to write zeroes to the affected
blocks, and thus clear errors.

Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>