os: _check_disk_write_cache: fix error handling
os: _check_disk_write_cache: fix error handling

Fix error handling; use popen instead of a temporary file.

Signed-off-by: Colin McCabe <7b35906f1d014e74a9dc8c913eb229119df647aa@dreamhost.com>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph