unlock a spinlock object
pthread_spin_unlock() function releases the spin
lock referenced by lock which was locked via the
pthread_spin_unlock() returns zero;
otherwise an error number is returned to indicate the error.
pthread_spin_unlock() will fail if:
- The value specified by lock is invalid.
- The lock is not owned by the calling thread.
pthread_spin_unlock() conforms to IEEE
Std 1003.1-2008 (“POSIX.1”).