Stop managed arrays more carefully.

If an array is being managed by mdmon, then just
write "inactive" to stop it, and let mdmon do the
final "clear".  This makes sure mdmon has a chance
to read the final state and update the metadata properly.

After writing "inactive" with use "ping_monitor" to synchronise
with mdadm, then STOP the array just in case it is still running,
else we will get into an infinite loop in "mdadm -Ss".

Signed-off-by: Neil Brown <neilb@suse.de>