libnvdimm: fix mishandled nvdimm_clear_poison() return value

Colin, via static analysis, reports that the length could be negative
from nvdimm_clear_poison() in the error case. There was a similar
problem with commit 0a3f27b9 "libnvdimm, namespace: avoid multiple
sector calculations" that I noticed when merging the for-4.10/libnvdimm
topic branch into libnvdimm-for-next, but I missed this one. Fix both of
them to the following procedure:

* if we clear a block's worth of media, clear that many blocks in
  badblocks

* if we clear less than the requested size of the transfer return an
  error

* always invalidate cache after any non-error / non-zero
  nvdimm_clear_poison result

Fixes: 82bf1037 ("libnvdimm: check and clear poison before writing to pmem")
Fixes: 0a3f27b9 ("libnvdimm, namespace: avoid multiple sector calculations")
Cc: Fabian Frederick <fabf@skynet.be>
Cc: Dave Jiang <dave.jiang@intel.com>
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>