block: send disk "change" event for rescan_partitions()

Userspace likes to get notified that the disk may have changed, when
rescan_partitions() is called after partitioning or media change. It will
make it possible to update the state of the disk with the "change" event,
before the following partition "add" events are handled.

Cc: David Zeuthen <david@fubar.dk>
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>