Don't reparent if a cutoff is pending

And update master->splitPointsCnt under lock
protection. Not stricly necessary because
single_bit() condition takes care of false
positives anyhow, but it is a bit tricky and
moving under lock is the most natural thing
to do to avoid races with "reparenting".

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>