Recursive lock all split point's chain

When we found a cut-off then lock all the split point chain,
not only current one to avoid races in case two threads running
on different split points where one is ancestor then the other,
find a beta cut-off at the same time, in this case we want only
one to call sp_update_pv().

No functional change.

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