os-prober: call dmraid -r *once*

os-prober: call dmraid -r *once*

os-prober calls dmraid -r -c unqualifed for every partition and then greps
the result. dmraid -r -c without a device list will scan every device
on the system. Every time. The results will not differ between runs.

This patch saves the results and performs the grep afterward.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Tweaked-by: Colin Watson <cjwatson@debian.org>