packetpool: signal waiter within lock

packetpool: signal waiter within lock

Needed for predictable scheduling. From pthread_cond_signal man page:

"The pthread_cond_signal() or pthread_cond_broadcast() 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 behaviour is required, then that mutex is locked
 by the thread calling pthread_cond_signal() or pthread_cond_broadcast()."