Kobject: convert drivers/md/md.c to use kobject_init/add_ng()

This converts the code to use the new kobject functions, cleaning up the
logic in doing so.

Cc: Neil Brown <neilb@suse.de>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>