annotate S_ithread_create create_destruct_mutex use

annotate S_ithread_create create_destruct_mutex use

If the pool is passed in, and the create_destruct_mutex unlocked also
on success (so that all the unlocks happen in S_ithread_create()),
the create_destruct_mutex use can be annotated.

threads.xs:757:9: warning: releasing mutex 'my_poolp->create_destruct_mutex' th
at was not held [-Wthread-safety-analysis]
        MUTEX_UNLOCK(&MY_POOL.create_destruct_mutex);

threads.xs:1000:9: warning: releasing mutex 'my_poolp->create_destruct_mutex' t
hat was not held [-Wthread-safety-analysis]
        MUTEX_UNLOCK(&MY_POOL.create_destruct_mutex);

threads.c:1319:1: warning: mutex 'my_poolp->create_destruct_mutex' is not held on every path through here [-Wthread-safety-analysis]
}
^