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>