[sanitizers] Optimize locking StackDepotBase for fork (#76280)

Locking StackDepotBase fully is very expensive, as 2^20 buckets needs to
be locked. Not locking, but only unlocking buckets, needed to be
unlocked to avoid deadlocks, increases a chance of data race, when the
value with same hash can be inserted into table twice, but one is lost.
However this is just a small additional memory usage by forked process.