close_sync() needs to set conf->next_resync to a large, but safe value
below MaxSector and use it to determine whether or not to set
start_next_window in wait_barrier()

Solution suggested by Neil Brown.
Reported-by: NNate Dailey <nate.dailey@stratus.com>
Tested-by: NXiao Ni <xni@redhat.com>
Signed-off-by: NJes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: NNeilBrown <neilb@suse.com>