dm raid: fix rebuild of specific devices by updating superblock

Update superblock when particular devices are requested via rebuild
(e.g. lvconvert --replace ...) to avoid spurious failure with the "New
device injected into existing raid set without 'delta_disks' or
'rebuild' parameter specified" error message.

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>