Grow: fix hang when growing a RAID5.

Since:

commit 84d11e6c6a3b827b2daa32e16303235ce33d49f5
Author: NeilBrown <neilb@suse.de>
Date:   Thu Aug 1 11:16:14 2013 +1000

    Grow: exit background thread cleanly on SIGTERM.

removed the setting of "sync_max" from abort_reshape() we need
to do it explicitly here.

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