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>