Grow: fix resent grow_continue breakage.

Grow: fix resent grow_continue breakage.

Commit 5e76dce1acd906e8fc8af04973c3a129cdc77fd6 changed
Grow_continue to assume a fork had already happened, so that
   mdadm --grow --continue

didn't fork.  This is good, but it means that if Grow_continue
is run from Assemble, then
  mdadm --assemble ....

can misbehave if the array was in the middle of a reshape.

So introduce finer control.  Grow_continue only assumes it has
already forked if run from "mdadm --grow --continue".

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