Do not return from idle_loop() with lock held

Do not return from idle_loop() with lock held

Master thread returns from idle_loop() when sp->cpus == 0,
but cpus is decremented by slave threads under sp->lock,
so it could happen that we return in split(), where we release
the split point, with sp->lock still held.

This patch guarantees that sp->lock is released when returning
to split().

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