If you make an array bigger but suppress resync of the new region with
  mdadm --grow /dev/mdX --size=max --assume-clean

then stop the array before anything is written to it, the effect of
the "--assume-clean" is lost and the array will resync the new space
when restarted.
So ensure that we update the metadata in the case.
Reported-by: NSebastian Riemer <sebastian.riemer@profitbricks.com>
Signed-off-by: NNeilBrown <neilb@suse.de>