sysvinit, upstart: use df -P when weighting new OSDs

This avoids parsing out the wrong value when a long device name makes df wrap
over two lines.

Fixes: #6699 Reported-by: Jan Harkes <jaharkes@cs.cmu.edu> Reviewed-by: Noah
Watkins <noah.watkins@inktank.com> Signed-off-by: Sage Weil
<sage@inktank.com>