Make reduction search code SMP-friendly

Make reduction search code SMP-friendly

In sp_search_pv() we do a LMR search using sp->alpha, at the end
we detect a fail high with condition (value > sp->alpha), but if
another thread has increased sp->alpha during our LMR search we
could miss to detect a fail high event becasue value will be equal
to old alpha and so smaller then new one.

This patch fixes this SMP-bug and changes also the non SMP versions
of the search to keep code style in sync.

Bug spotted by Bruno Causse.

No functional change (for single CPU case)

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