blk: check the return value of pthread_mutex_lock()

Andy, and what about (already existing in code) checking the return value of:
pthread_mutex_unlock() in blk.c: 99, 170
and
pthread_rwlock_unlock() in log.c: 283, 381, 462, 485, 519, 568 ?
Should I remove checking the return value or leave them as-is?