Signal td->free_cond with the associated mutex held

Signal td->free_cond with the associated mutex held

Calling pthread_cond_signal() or pthread_cond_broadcast() without
holding the associated mutex can lead to missed wakeups. Hence ensure
that td->io_u_lock is held around pthread_cond_signal(&td->free_cond)
calls. A quote from the POSIX spec
(http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_broadcast.html):

"The pthread_cond_broadcast() or pthread_cond_signal() functions may be
called by a thread whether or not it currently owns the mutex that
threads calling pthread_cond_wait() or pthread_cond_timedwait() have
associated with the condition variable during their waits; however, if
predictable scheduling behavior is required, then that mutex shall be
locked by the thread calling pthread_cond_broadcast() or
pthread_cond_signal()."

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>