libnvdimm, tools/testing/nvdimm: fix 'ars_status' output buffer sizing

Use the output length specified in the command to size the receive
buffer rather than the arbitrary 4K limit.

This bug was hiding the fact that the ndctl implementation of
ndctl_bus_cmd_new_ars_status() was not specifying an output buffer size.

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