libnvdimm: check and clear poison before writing to pmem

We need to clear any poison when we are writing to pmem. The granularity
will be sector size. If it's less then we can't do anything about it
barring corruption.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
[djbw: fixup 0-length write request to succeed]
Signed-off-by: Dan Williams <dan.j.williams@intel.com>