Enable error checking for the mutex that serializes overlapping I/O

Enable error checking for the mutex that serializes overlapping I/O

Since it is nontrivial to verify that pthread_mutex_lock() and unlock
calls are paired for the overlap_check mutex, enable runtime verification.
The PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP initializer causes
pthread_mutex_lock() to fail with EDEADLK if the mutex was already locked
and causes pthread_mutex_unlock() to fail with EPERM if the mutex is owned
by another thread.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>