BUG/MAJOR: fd/threads, task/threads: ensure all spin locks are unlocked

BUG/MAJOR: fd/threads, task/threads: ensure all spin locks are unlocked

Calculate if the fd or task should be locked once, before locking, and
reuse the calculation when determing when to unlock.

Fixes a race condition added in 87d54a9a for fds, and b20aa9ee for tasks,
released in 1.9-dev4. When one thread modifies thread_mask to be a single
thread for a task or fd while a second thread has locked or is waiting on a
lock for that task or fd, the second thread will not unlock it.  For FDs,
this is observable when a listener is polled by multiple threads, and is
closed while those threads have events pending.  For tasks, this seems
possible, where task_set_affinity is called, but I did not observe it.

This must be backported to 1.9.